at76c50x-usb: fix use after free on failure path in at76_probe()

After commit 174beab7 ("at76c50x-usb: Don't perform DMA from stack memory")
at76_delete_device() and usb_put_dev() are called both
if at76_init_new_device() fails in at76_probe().
But at76_delete_device() does usb_put_dev(priv->dev) itself
that means double usb_put_dev().

The patch avoids the problem by moving usb_put_dev() from
at76_delete_device() to at76_disconnect().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: John W. Linville <linville@tuxdriver.com>