Bluetooth: Add support for reading encryption key size for BR/EDR

Make use of "Read Encryption Key Size" HCI command to read real
encryption key size for BR/EDR connection instead of assuming 16
bytes key size.

< HCI Command: Read Encryption Key Size (0x05|0x0008) plen 2
        Handle: 11
> HCI Event: Command Complete (0x0e) plen 7
      Read Encryption Key Size (0x05|0x0008) ncmd 1
        Status: Success (0x00)
        Handle: 11
        Key size: 16

Change-Id: I372028ce73115d34aedbb6b5ce420144844cfce0
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>