Changes

Check for protocol extension bit in message type at a central location

Bits 8 and 7 of message type can be used as sequence number or have
specific default values. The added check will verify the bits according
to the prococol descriptor and Release version.

The Rules for these bits are described in GSM TS 124.007, clause
11.2.3.2.1 for Release <= 98 and 11.2.3.2.2 for Release >= 99.

There is also no need to clear bit 7 in message type for these
protocols, as it is done already in gsm48_hdr_msg_type().

Change-Id: Iae41bc6a2e9fd85583509b6c6154dd5a935fb5df