04.08: add inline funcs for pdisc + msg type bitmasks

Add inline functions for both release <= 98 and release >= 99 as well as a
default define. Use the release 98 by default since the current code base uses
the r98 bitmasks.

These inline functions relieve callers of the decision on masking bits of the
protocol discriminator and message type octets.

Also add a define for the protocol discriminator extension to one octet length
(GSM48_PDISC_EXTEND).

Apply new pdisc function in gsm0480.c.

[hfreyther: Make the hdr param const]
2 files changed