ath9k: cleanup a min_t() cast

If the firmware was over 2G, it would cause memory corruption and the
system would die here.  Obviously we all know the firmware isn't going
to be that large but static checkers get upset.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>