mwifiex: Prevent memory corruption handling keys

The length of the key comes from the network and it's a 16 bit number.  It
needs to be capped to prevent a buffer overflow.

Fixes: 5e6e3a92 ("wireless: mwifiex: initial commit for Marvell mwifiex driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Ganapathi Bhat <ganapathi.bhat@nxp.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200708115857.GA13729@mwanda