brcmfmac: fix brcmf_fws_add_interface() for USB devices

USB devices rely on queuing functionality provided by the fwsignal
module regardless the mode fwsignal is operating in. For this some
data structure needs to be reserved which is tied to the interface,
which is done by brcmf_fws_add_interface(). However, it checks the
mode. Replace that by checking result from brcmf_fws_queue_skbs().
Otherwise the driver will crash in a null pointer dereference when
data is transmitted on the interface.

Fixes: fc0471e3 ("brcmfmac: ignore interfaces when fwsignal is disabled")
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>