gprs_llc: Correctly refuse any ABM command (SABM, DISC) with DM

According to Section 6.4.1.4 of 3GPP TS 04.64
	The DM unnumbered response shall be used by an LLE to report to
	its peer that the LLE is in a state such that ABM operation
	cannot be performed. An LLE shall transmit a DM response to any
	valid command received that it cannot action.

Closes: OS#3953
Change-Id: Ie8b8e16d5a68f19f21dc4fdb5703c8a794e0173c