usb: misc: uss720: properly clean up reference in uss720_probe()

If get_1284_register() fails, the usb device reference count is
incorrect and needs to be properly dropped before returning.

Cc: stable <stable@kernel.org>
Assisted-by: gkh_clanker_2000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>