ar5523: Fix sparse endianness warnings

__be32 variables where used a little careless leading to sparse warnings.
Treat them a little more gentle.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>