mac80211: michael.c use kernel-provided infrastructure

Replace private implementation of bit rotation and unaligned access helpers
with kernel-provided implementation.

Fold xswap helper in its one usage in the michael_block macro.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Cc: "John W. Linville" <linville@tuxdriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>