brcmfmac: take save&restore memory into account for SDIO shared info

The firmware provides pointer to SDIO shared information at end of
RAM during firmware initialization. End of RAM is obviously determined
by the actual ram size, but part of that may be used for save&restore
memory. In that case another location in RAM will hold the pointer.

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: Kalle Valo <kvalo@codeaurora.org>