uhd: flush receive buffer should return true on timeout
uhd: flush receive buffer should return true on timeout

Receive buffer flush should continue to read until
either the desired number of packets has been read or
timeout, which means that the buffer has been emptied.
These are expected behaviours and should return true.

Ignore errors at this stage as the data and associated
metadata can be considered garbage and not worth
reporting. Actual error conditions will be caught
further downstream when useful data comes in.

Signed-off-by: Thomas Tsou <ttsou@vt.edu>

git-svn-id: 504cad4147f1e500be0a169b9f46375eaf875953@2659 19bc5d8c-e614-43d4-8b26-e1612bc8e597

kkroo/osmo-trx,ttsou/osmo-trx,kkroo/osmo-trx,pothosware/osmo-trx,pothosware/osmo-trx,kkroo/osmo-trx,osmocom/osmo-trx,telenoobie/osmo-trx,osmocom/osmo-trx,pothosware/osmo-trx,ttsou/osmo-trx,pothosware/osmo-trx,osmocom/osmo-trx,ttsou/osmo-trx,ttsou/osmo-trx,kkroo/osmo-trx,osmocom/osmo-trx,telenoobie/osmo-trx,telenoobie/osmo-trx,telenoobie/osmo-trx