brcmfmac: replace brcmf_sdcard_cfg_read with brcmf_sdio_regrb

brcmfmac: replace brcmf_sdcard_cfg_read with brcmf_sdio_regrb

Use the newly introduced brcmf_sdio_regrb to replace
brcmf_sdcard_cfg_read as part of the SDIO WiFi dongle register
access interface clean up.

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>