brcmfmac: always use worker thread for tx data.

When fw signalling is disabled tx is sent immediately. Using
queues and worker thread allows usb to do synchronous autopm. This
patch makes fws use queues and worker thread even if signalling is
not supported by FW or not enabled.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>