ath9k_hw: min_t() casts u32 to int

The code here treats very large values of "limit" as less than
MAX_POWER_RATE because of the cast to int.  We should do the compare
as u32 instead.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>