rt2x00: Reduce tx status reading timeout

When no TX status was available, the default timeout
of 20ms is a bit high. The frame is highly likely already
send out, so the TX status should be available within
only a few milliseconds.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>