airo: use simple_write_to_buffer

Simplify write file operation for /proc files by using
simple_write_to_buffer().

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: "John W. Linville" <linville@tuxdriver.com>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>