libertas: Fix sd8686 firmware reload

For the SD8686, we cannot rely on the scratch register to read the firmware
load status, because the same register is used for storing RX packet length.
Broaden the check to account for this.

The module can now be unloaded/reloaded successfully.

Based on the implementation from libertas_tf.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: Steve deRosier <steve@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>