mwifiex: potential corruption in mwifiex_update_uap_custom_ie()

ap_custom_ie is a struct mwifiex_ie_list which is quite different and
also larger than struct mwifiex_ie.  It's a difference between 4196
bytes and 262.

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