brcmfmac: clarify struct brcmf_sdio_dev::func[0] reference

The struct brcmf_sdio_dev contains array of sdio functions that
are used in the driver. However, during probe func[0] entry was
assigned to the function 1 reference. This was corrected upon
doing the actual I/O access. This patch makes it more clear by
creating the func[0] entry properly and use it as is during
I/O access.

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>