mwifiex: do not re-associate when already connected

In managed mode if the driver is getting a re-associate command
from cfg80211, driver deauthenticates with the AP internally and
sends a disconnected event to cfg80211 before completion of its
association process. The disconnected event then modifies the
SSID length as wdev->ssid_len = 0. So, upon receiving the connect
result event from driver, cfg80211 is unable to get that BSS from
the device's BSS list and generates the following WARN_ON message.

WARNING: CPU: 0 PID: 857 at net/wireless/sme.c:658
         __cfg80211_connect_result+0x3a6/0x3e0 [cfg80211]()

Avoid re-association while the device is already associated to a
network. Also remove the internal deauthentication from the
association path.

Signed-off-by: Ujjal Roy <royujjal@gmail.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>