fix bogus error check in gprs_sndcp_comp_create()

The function gprs_sndcp_get_compression_class() returns -EINVAL
upon error, not -1, so an existing assertion would never trigger.

Instead, check for the values we want first (PROTOCOL_COMP or
DATA_COMP) and assert(false) in case the returned value doesn't
match either of these.

Found by: Neels

Change-Id: I8444c1ed052707c76a979fb06cb018ac678defa7
1 file changed