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>
diff --git a/drivers/net/wireless/rt2x00/rt2x00debug.c b/drivers/net/wireless/rt2x00/rt2x00debug.c
index 74f0d67..daf7f95 100644
--- a/drivers/net/wireless/rt2x00/rt2x00debug.c
+++ b/drivers/net/wireless/rt2x00/rt2x00debug.c
@@ -480,6 +480,9 @@
 	if (index >= debug->__name.word_count)			\
 		return -EINVAL;					\
 								\
+	if (length > sizeof(line))				\
+		return -EINVAL;					\
+								\
 	if (copy_from_user(line, buf, length))			\
 		return -EFAULT;					\
 								\