wifi: libertas: Fix possible refcount leak in if_usb_probe()

usb_get_dev will be called before lbs_get_firmware_async which means that
usb_put_dev need to be called when lbs_get_firmware_async fails.

Fixes: ce84bb69 ("libertas USB: convert to asynchronous firmware loading")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220620092350.39960-1-hbh25y@gmail.com
Link: https://lore.kernel.org/r/20220622113402.16969-1-colin.i.king@gmail.com