mwifiex: fix potential integer truncation

At some places, ie length is truncated from u16 to u8 while
storing it to driver's internal variable. This patch fixes
the problem.

Signed-off-by: chunfan chen <jeffc@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>