brcmfmac: rework single packet transfers

The function brcmf_sdio_sglist_rw() does a different code path
when packet queue length is 1. Move this to a separate function
reducing overhead in the calling context.

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>