USB: serial: console: move mutex_unlock() before usb_serial_put()

While in current version there is no use-after-free as USB serial
core holds another reference when the console is registered, we
should better unlock before dropping the reference in
usb_console_setup().

Fixes: 7bd032dc ("USB serial: update the console driver")
Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Johan Hovold <johan@kernel.org>