mwifiex: fix 'smatch' warning in preparing key_material cmd

The key length can be 32 bytes for TKIP and 16 bytes for AES_CMAC.
'smatch' warns on memcpy using key_len variable to copy data to
a 16 bytes buffer. Use fixed length to avoid the warning.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>