staging: greybus: loopback: use gb_loopback_async_wait_all don't spin

Currently the greybus-loopback thread logic spins around waiting for
send_count == iteration_max which on real hardware doesn't make a
difference to us but in simulation is excruciatingly slow, anti-social and
bad manners. Use the existing gb_loopback_async_wait_all() function to gate
continuing when the send_count == iteration_max and go to sleep until
there's something worthwhile to-do.

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>