mwifiex: add NULL checks in driver unload path

If driver load is failed, sometimes few pointers may remain
uninitialized ex. priv->wdev, priv->netdev, adapter->sleep_cfm
This will cause NULL pointer dereferance while unloading the
driver.

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