rt2x00: fix "buffer size not provably correct" build warning

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: Arnaud Patard <arnaud.patard@rtp-net.org>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>