libertas: add auto auth mode feature

Auto auth mode is enabled by default. If user doesn't specify the
auth mode, while association driver will first try with open mode
and then with shared key mode. If user specifies an auth mode,
auto auth is disabled and driver will not try association with
another auth mode.

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>