rt2x00: Move start() and stop() handlers into rt2x00lib.c

suspend & resume was broken since it called rt2x00mac_start()
and rt2x00mac_stop() which would fail to execute because the
DEVICE_PRESENT flag was not set.

Move the start and stop handlers into rt2x00lib.c which are called
from rt2x00mac_start() and rt2x00mac_stop() after they have checked
the DEVICE_PRESENT flag, while suspend and resume handlers can
directly call those functions.

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