carl9170: reduce channel change delay

By removing two "safety" msleeps (and an echo nop), the
channel change delay is effectively halved. Previously,
the delay could be as long as 260 ms and the device
could not go off-channel without risking to miss the
next DTIM beacon [interval ~307 ms].

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>