at76c50x-usb: avoid double usb_put_dev() after downloading internal firmware in at76_probe()

There is no need in usb_put_dev() if at76_load_internal_fw() succeed.

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

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>