USB: serial: add Fintek F81232 usb to serial driver

This is the first cut at a driver for the Fintek F81232 USB to serial
single port converter.  This provides the initial framework for the
device, and some data can move through it, but no line settings are
handled, so it's not that useful yet.  It does give people a starting
place to work from.

Thank to Fintek for providing samples and specifications, without which,
this driver would have never been able to be written.

Cc: Amanda Ying <Amanda_Ying@fintek.com.tw>
Cc: Tom Tsai <Tom_Tsai@fintek.com.tw>
Cc: <linux-usb@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>