brcmfmac: use configurable sdio bus header length for tx packet

Host tx glomming require an extended hardware sdio bus header to store
information for dongle. Introduce a variable in struct brcmf_sdio to replace
macro SDPCM_HDRLEN

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend van Spriel <arend@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>