sigproc: Match differential GMSK start/end bits to tail bits
sigproc: Match differential GMSK start/end bits to tail bits

Invert the initial and trailing diffential bits in the 4 sps
GMSK modulator to fix power-time mask irregularity at start
and end of burst. GSM tail bits are always zero, so we can
safely use fixed bit values.

Signed-off-by: Tom Tsou <072b11b861b7a066437a0a0cead1208995732a66@ettus.com>

osmocom/osmo-trx,ttsou/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx,ttsou/osmo-trx,osmocom/osmo-trx,ttsou/osmo-trx,ttsou/osmo-trx