libertas: read SD8688 firmware status from new register

The scratch pad register is used to store firmware status after
firmware is downloaded and initialized. After firmware status is
verified OK, the same register is used to store RX packet length.
Hence the firmware status code is no longer valid afterwards.

SD8688 firmware introduces a new register for firmware status
which will never be overwritten.

Also add scratch_reg variable to if_sdio_card structure and
initialize it based on the model of the card during probe.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>