mwifiex: check for mfg_mode in add_virtual_intf

If driver is loaded with 'mfg_mode' enabled, then the sending
commands are not allowed. So, skip sending commands, to firmware
in mwifiex_add_virtual_intf if 'mfg_mode' is enabled.

Fixes: 7311ea85 ("mwifiex: fix AP start problem for newly added interface")
Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>