brcmfmac: Remove brcmf_sdiod_addrprep()

This function has become trivial enough that it may as well be pushed into
its callers, which has the side-benefit of clarifying what's going on.

Remove it, and rename brcmf_sdiod_set_sbaddr_window() to
brcmf_sdiod_set_backplane_window() as it's easier to understand.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>