libertas: switch to a waitqueue and timer for handling USB firmware load

No need to busy-wait, even if we did have a 100ms delay in the loop.
This makes it easier to support the new 'firmware ready' event which is
in the new firmware, too.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>