mwifiex: fix regression in WEP security mode

Htcapinfo is unnecessarily sent in assoc request in WEP security due
to a regression introduced by commit 2be50b8df53 (mwifiex: remove
redundant encryption_mode mapping).
The issue is fixed in this patch.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>