mwifiex: remove redundant encryption_mode mapping

remove MWIFIEX_ENCRYPTION_MODE_ and use WLAN_CIPHER_SUITE_
macros directly

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>