drivers/char: add RX support to the XHCI driver

drivers/char: add RX support to the XHCI driver

Add another work ring buffer for received data, and point IN TRB at it.
Ensure there is always at least one pending IN TRB, so the controller
has a way to send incoming data to the driver.
Note that both "success" and "short packet" completion codes are okay -
in fact it will be "short packet" most of the time, as the TRB length is
about maximum size, not required size.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Jan Beulich <jbeulich@suse.com>