libertas: fix 8686 firmware loading regression

The 'ready' condition was incorrectly evaluated which sometimes lead to
failures loading the second-stage firmware on 8686 devices.

(This was introduced in "libertas: consolidate SDIO firmware wait code".
-- JWL)

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>