brcmfmac: Remove {r,w}_sdreg32

Remove yet another IO function from the code and replace with one
that already exists.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
[arend: keep address calculation, ie. (base + offset) in one line]
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>