rt2x00 debugfs interface doesn't check the size of the data coming
from userspace, leading to a build warning. Fix That.

v2: return EINVAL if input is too long
Signed-off-by: NArnaud Patard <arnaud.patard@rtp-net.org>
Acked-by: NIvo van Doorn <IvDoorn@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>