brcmfmac: do not proceed if fail to download nvram to dongle

Nvram contains critical initialization parameter for firmware to run. Host
driver should not proceed if nvram fails to be downloaded to dongle.

Reviewed-by: Piotr Haber <phaber@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>