bugfix: workaround a problem with timestamp and LMSSDR.

We have a strange problem with the timestamp of the very
first packets received from the LimeSDR that leads to
a complete failure of the eNB.

A simple workaround is to discard the first RX packet
just after a "start" of the device.

We also change the value of "tx_sample_advance" to a more
reasonable value. The problem of timestamp was "solved"
differently in the past and at the time we needed to put
a much higher value.

This may have to be refined at some point.