Bluetooth: SMP: Add support for encryption key size reduction

This allows to pair with devices that use reduced encryption key size.
Encryption key size is stored with keys for future use. LTKs are kept
in full form (16 bytes) and are reduced only when used.

As master:
< ACL Data TX: Handle 64 flags 0x00 dlen 11
      SMP: Pairing Request (0x01) len 6
        IO capability: NoInputNoOutput (0x03)
        OOB data: Authentication data not present (0x00)
        Authentication requirement: Bonding, No MITM, Legacy,
                                    No Keypresses (0x01)
        Max encryption key size: 16
        Initiator key distribution: EncKey Sign (0x05)
        Responder key distribution: EncKey IdKey Sign (0x07)
> ACL Data RX: Handle 64 flags 0x02 dlen 11
      SMP: Pairing Response (0x02) len 6
        IO capability: KeyboardDisplay (0x04)
        OOB data: Authentication data not present (0x00)
        Authentication requirement: No bonding, No MITM, Legacy,
                                    No Keypresses (0x00)
        Max encryption key size: 7
        Initiator key distribution: <none> (0x00)
        Responder key distribution: <none> (0x00)
...
< HCI Command: LE Start Encryption (0x08|0x0019) plen 28
        Handle: 64
        Random number: 0x0000000000000000
        Encrypted diversifier: 0x0000
        Long term key: df3cff52a981d6000000000000000000

As slave:
> ACL Data RX: Handle 64 flags 0x02 dlen 11
      SMP: Pairing Request (0x01) len 6
        IO capability: KeyboardDisplay (0x04)
        OOB data: Authentication data not present (0x00)
        Authentication requirement: No bonding, No MITM, Legacy,
                                    No Keypresses (0x00)
        Max encryption key size: 7
        Initiator key distribution: <none> (0x00)
        Responder key distribution: <none> (0x00)
< ACL Data TX: Handle 64 flags 0x00 dlen 11
      SMP: Pairing Response (0x02) len 6
        IO capability: NoInputNoOutput (0x03)
        OOB data: Authentication data not present (0x00)
        Authentication requirement: No bonding, No MITM, Legacy,
                                    No Keypresses (0x00)
        Max encryption key size: 16
        Initiator key distribution: <none> (0x00)
        Responder key distribution: <none> (0x00)
...
> HCI Event: LE Meta Event (0x3e) plen 13
      LE Long Term Key Request (0x05)
        Handle: 64
        Random number: 0x0000000000000000
        Encrypted diversifier: 0x0000
< HCI Command: LE Long Term Key Request Reply (0x08|0x001a) plen 18
        Handle: 64
        Long term key: 701b431a9e17bb000000000000000000

Change-Id: Ibc70aa01c040aff0d39410d273d6880d35aa5ae0
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>