mwifiex: don't try to associate when bss_mode is not STA

We have blocked association attempts on interfaces configured in
AP and AD-HOC modes. P2P mode should be blocked too.

Furthermore, an error code must be returned if we are unable to
associate.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>