RSL: Reject RLL messages for lchans that are not active yet
RSL: Reject RLL messages for lchans that are not active yet

The Radio Link Layer (RLL) messages only make sense when a given
logical channel is active.  If it isn't active, let's reject the
messages with an RSL ERROR REPORT with cause "Message sequence error",
wich according to spec means:

"A message with an existing message type which is not possible according
 to the specification and to the state of the BTS is erroneous."

Related: OS#3750
Change-Id: I68dbb622aeaee657471664cdc0b69c2ac316d77e

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts