caif: Add debug connection type for CAIF.

Added new CAIF protocol type CAIFPROTO_DEBUG for accessing
CAIF debug on the ST Ericsson modems.

There are two debug servers on the modem, one for radio related
debug (CAIF_RADIO_DEBUG_SERVICE) and the other for
communication/application related debug (CAIF_COM_DEBUG_SERVICE).

The debug connection can contain trace debug printouts or
interactive debug used for debugging and test.

Debug connections can be of type STREAM or SEQPACKET.

Signed-off-by: Sjur Braendeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>