rt2x00: Block set_key() during suspend/shutdown

mac80211 will call set_key() when the device is
shutting down. When the device is unplugged the
keys will be lost automatically due to the power
loss. When the device is not plugged but the module
is only unloaded the keys can remain in the device
hardware, when the module is loaded the keys will
be cleaned up during initialization.

This should prevent the problem reported by Johannes Berg,
where unplugging the device while suspended resulted in
a NULL pointer error during set_key() which was
caused because of the CSR base address being freed.

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