mwifiex: convert to use le16_add_cpu()

Convert cpu_to_le16(le16_to_cpu(E1) + E2) to use le16_add_cpu().

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: John W. Linville <linville@tuxdriver.com>