Fix receive path hangs if another thread closes down the hackrf
receive whilst this buffer receive function is waiting to be
woken up.
Now:
* Sleep for up to 100ms each time waiting for the cond to be kicked;
* Check whether streaming is still enabled each time rather than
only when the function is entered.
This fixes hangs where consumers like gqrx via gnuradio
will do a stop_rx/start_rx very quickly to change something, and
the buffer receive path is waiting for a buffer.
Signed-off-by: Eric Wild <ewild@sysmocom.de>