mwifiex: remove unnecessary struct mwifiex_802_11_ssid

Use struct cfg80211_ssid available in include/net/cfg80211.h
instead of having similar definition in driver.

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>