usb: gadget: u_serial: add .get_icount() support

Add icount support for the transmitting and receiving characters. This
will make the tty LED trigger work with the ttyGS devices.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Link: https://lore.kernel.org/r/20240730193840.2580358-1-mwalle@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>