bluetooth: hci_uart: Allocate 65 bytes for L2CAP packets

With LE Secure Connections the longest packet we require is 65 bytes
long: 64 bytes of Public Key data + 1 byte SMP opcode.
Extend the size of the L2CAP pool blocks so that they can fit such
packets and therefore one can use 70-byte
CONFIG_BLUETOOTH_CONTROLLER_TX_BUFFER_SIZE to achieve LE Secure
Connections pairing without HCI fragmentation.

Change-id: I2f11b4277ec5a24ceba79bccc7d84c64065b6e84
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>