Changes

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

The protocols MM, CC, SS, GCC, BCC and LCS have an extension bit 8 for
future prococol extensions. It must be 0 with the current
specification. This is now checked at a central location, rather than
checked at each protocol receiver function. It was checked only in CC,
but without any effect, because the bit was cleared already.

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