Bluetooth: A2MP: Process A2MP messages

Implement basic processing for AMP Manager Protocol (A2MP).

Example below shows processing unrecognized command.
...
> ACL data: handle 11 flags 0x02 dlen 12
    A2MP: code 0x00 ident 3 len 0
< ACL data: handle 11 flags 0x00 dlen 14
    A2MP: Command Reject: reason (0) - Command not recognized
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>