brcm80211: fmac: fix SDIO function 0 register r/w issue

SDIO stack doesn't have a structure for function 0. The structure
pointer stored in card->sdio_func[0] is actually for function 1.
With current implementation the register read/write is applied to
function 1. This pathch fixes the issue.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Florian Fainelli <florian@openwrt.org>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>