mwifiex: return success in set_default_key for WPA/WPA2

mwifiex: return success in set_default_key for WPA/WPA2

When testing wpa_supplicant with 'nl80211' driver to connect
to an AP with WPA/WPA2 security, we notice the followings:

1) add_key is called firstly with the key from cfg80211
2) set_defaut_key is called next

set_default_key() is specific to WEP keys and should not be
called in case of WPA/WPA2 security. The set_default_key()
won't be called if wpa_supplicant uses "-Dwext" option,
but it's been called if "-Dnl80211" option is specified.

We can fix this issue by adding a check to return from
set_default_key() if WEP key is not configured.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Marc Yang <yangyang@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>