brcmfmac: tag packet in the netdev transmit callback

Transmit packets needs to be tagged in order to receive a tx status
feedback from the firmware. Determine the tag in the netdev transmit
callback instead of determining the tag just before transfer to the
device. This reduces the number of exception flows and hence makes
the driver code simpler.

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