mwifiex: reset encryption mode flag before association

Recent commit
"mwifiex: clear previous security setting during association"
fixes association failure problems observed in some corner cases
by clearing previous security setting before each association.

We should reset encryption mode flag as well. This patch takes care
of it.

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>