USB: serial: make usb_serial_driver::write_room return uint

Line disciplines expect a positive value or zero returned from
tty->ops->write_room (invoked by tty_write_room). Both of them are being
updated to return an unsigned int. Switch also
usb_serial_driver::write_room and all its users.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
[ johan: amend commit message, drop unrelated comment change ]
Signed-off-by: Johan Hovold <johan@kernel.org>