Merge tag 'usb-serial-4.18-rc1' of https://git.kernel.org/pub/scm/linux/kernel/git/johan/usb-serial into usb-next

Johan writes:

USB-serial updates for v4.18-rc1

Here are the USB-serial updates for 4.18-rc1, including:

 - support for hardware-assisted XON/XOFF output flow control for pl2303
 - fix for a long-standing IXON/IXOFF mixup in ftdi_sio
 - blacklist of two apparently unused dwm-158 modem interfaces that
   confused some user space daemon (option)
 - add missing const to a tty helper currently used by USB serial only

Included are also various clean ups.

All have been in linux-next with no reported issues.
Signed-off-by: Johan Hovold <johan@kernel.org>