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

drivers/net/wireless/rt2x00/rt2x00debug.c

@@ -480,6 +480,9 @@ static ssize_t rt2x00debug_write_##__name(struct file *file,	\
 	if (index >= debug->__name.word_count)			\
 		return -EINVAL;					\
 								\
+	if (length > sizeof(line))				\
+		return -EINVAL;					\
+								\
 	if (copy_from_user(line, buf, length))			\
 		return -EFAULT;					\
 								\