bsc_vty: Don't allow timers of zero (0)

It typically doesn't make sense to configure any of the GSM RR timer
to 0 (Seconds).  In fact, accidentially configuring any of the timers
to zero might have severe side effects, such as "stuck channels"
described in https://osmocom.org/issues/2380

Change-Id: I517828f2f0c80ec01cb63648db2626f17a67fe57
1 file changed