mwifiex: fix unconditional error return in .add_virtual_intf callback

The commit 7311ea85 ("mwifiex: fix AP start problem for newly added
interface") attempted to fix an issue when a new AP interface is added.

But the patch didn't check the return value of the functions doing the
firmware calls and returned an error even if the functions didn't fail.

This prevents the network device to be registered properly, so fix it.

Fixes: 7311ea85 ("mwifiex: fix AP start problem for newly added interface")
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>