brcmfmac: rework SDIO register access functions

brcmfmac: rework SDIO register access functions

The logic in the SDIO register access functions was hard to
read and contained a lot of conditional code path. This rework
attempts to clean it up.

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>