staging: ath6kl: Fixing a NULL pointer exception

The driver was dereferencing a NULL pointer because of the device instance
being registered via the set_wiphy_dev() function. The function
ar6000_avail_ev() was passing the argument as NULL instead of using the one
returned by the MMC stack through the probe callback.

Signed-off-by: Vipin Mehta <vmehta@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>