mac80211: validate SIOCSIWPOWER arguments better

Don't accept any arguments we don't handle, and return error codes
instead of using an uninitialised stack value.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>