ath9k: Fix write callback of 'debug' which configures debug mask

Handle error condition on copy_from_user() properly and
make sure a NUL terminated char[] is sent to strict_strtoul()
for proper conversion.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>