Changes

trx: Fix tx/rx-sps 4 for USRP1 devices

The existing code sets the pingOffset variable to -7231(269 - 7500)
with tx-sps = 4. The (apparent) correct value for pingOffset for
tx/rx-sps 4 is 272.

While I'm not completely sure how this patch works to fix the increased
sps value, my theory is that the original OpenBTS TRX  only supported
a split sps setup(tx-sps=4, rx_sps=1 for sps=4), which created
differing decimation rates between Tx and Rx on the board.
In this case, apparently you needed to account for an advance in the
timestamp for a correct output.

With tx_sps == rx_sps, the decimation/sample rates will be the same,
and(I guess) this advance doesn't need to be accounted for.

Also changed a line in the initialization of the Rx end of the USRP
device to use the correct decimation rate when tx/rx-sps == 4.

Since osmo-trx won't allow nonequal values for tx-sps and rx-sps, I
commented out the logic that selected pingOffset based on sps.
I left it visible in the file for reference however.

Tested & working on the URAN-1, a USRP1 clone designed for GSM.

Change-Id: I3b08c23a3e7d4a226cd23b9fe42558f596a63257