brcmfmac: check brcmf_bus_get_memdump result for error

This method may be unsupported (see: USB bus) or may just fail (see:
SDIO bus).
While at it rework logic in brcmf_sdio_bus_get_memdump function to avoid
too many conditional code nesting levels.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>