brcmfmac: remove function brcmf_sdcard_regfail

The new interface brcmf_sdio_regr/w provides result of access
attempts. It is no longer necessary to use dedicated variable
and function to provide enquiry for failure.

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>