brcmfmac: introduce unified register access interface for SDIO

Both brcmf_sdcard_cfg_read/write and brcmf_sdcard_reg_read/write
are used as interface functions for register access of SDIO WiFi
dongle. A unified interface brcmf_sdio_regr/w is introduced in
this patch in order to simplify the interface and keep the
complexity within the lower layer.

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: John W. Linville <linville@tuxdriver.com>