At least writerProcessWaitingQueue() checks for the exact number of
samples written. In many other places, we check that the return is
non-zero.

The usrp-tx-write-thread functionality returned 0, which triggers
various problems. Return the number of samples, as in the "direct write"
case (if-block before else-block).
Co-authored-by: Sagar Arora <sagar.arora@openairinterface.org>