usb: gadget: u_serial: Remove old tasklet comments

Update old comments as of 8b4c62ae

 (usb: gadget: u_serial: process RX
in workqueue instead of tasklet).

Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Link: https://lore.kernel.org/r/20210119001321.127750-1-dave@stgolabs.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>