brcmfmac: fix brcmf_sdio_txpkt_prep() for host without sg support

brcmfmac: fix brcmf_sdio_txpkt_prep() for host without sg support

When running on a host controller that does not support scatter-gather
transfers the function brcmf_sdio_txpkt_prep() should not add tail
padding buffers.

Reviewed-by: Franky Lin <frankyl@broadcom.com>
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>