brcmfmac: add host tx glomming support

New WiFi full dongle supports receiving chained packets in one command
through the SDIO bus. This patch adds the support on the host side to
send chained packets.
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>