rt2x00 : bail out of regbusy_read if device is removed.

platform rfkill is async thus we may try to read while the device is
already off.

Signed-off-by: Alban Browaeys <prahal@yahoo.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <ivdoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>