usb: gadget: u_serial: process RX in workqueue instead of tasklet

Switch RX processing from tasklet to (delayed) work queue. This allows
receiver more room to process incoming data and prevents flood of
"ttyGS0: RX not scheduled?" messages on HS receive on slow CPU.

A side effect is 2.4MB/s zmodem transfer speed (up from 1.8MB/s)
on my test board.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>