usb: gadget: allow serial gadget console on other configs

USB gadget serial console works on functions other than the legacy
configurations. Let the user enable it when using any function that
uses the serial utilities.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@cascardo.eti.br>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>