osmo-bts-trx: Relax validation to allow TRX data bursts without padding
osmo-bts-trx: Relax validation to allow TRX data bursts without padding

Original OpenBTS transcievers add 2 bytes of padding to the end of data
bursts, having in total 158 bytes. As those two extra bytes are being
ignored after the initial validation, let's relax this validation a bit
in order to accept transcievers that decide no to send these two extra
bytes.

Change-Id: I94c3cb160bfed0ba9c41ed7ef5f8d8a65b81ad07

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