USB: ftdi_sio: add support for FT-X series devices

Add PID 0x6015, corresponding to the new series of FT-X chips
(FT220XD, FT201X, FT220X, FT221X, FT230X, FT231X, FT240X).  They all
appear as serial devices, and seem indistinguishable except for the
default product string stored in their EEPROM.  The baudrate
generation matches FT232RL devices.

Tested with a FT201X and FT230X at various baudrates (100 - 3000000).

Sample dmesg:
    ftdi_sio: v1.6.0:USB FTDI Serial Converters Driver
    usb 2-1: new full-speed USB device number 6 using ohci_hcd
    usb 2-1: New USB device found, idVendor=0403, idProduct=6015
    usb 2-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
    usb 2-1: Product: FT230X USB Half UART
    usb 2-1: Manufacturer: FTDI
    usb 2-1: SerialNumber: DC001WI6
    ftdi_sio 2-1:1.0: FTDI USB Serial Device converter detected
    drivers/usb/serial/ftdi_sio.c: ftdi_sio_port_probe
    drivers/usb/serial/ftdi_sio.c: ftdi_determine_type: bcdDevice = 0x1000, bNumInterfaces = 1
    usb 2-1: Detected FT-X
    usb 2-1: Number of endpoints 2
    usb 2-1: Endpoint 1 MaxPacketSize 64
    usb 2-1: Endpoint 2 MaxPacketSize 64
    usb 2-1: Setting MaxPacketSize 64
    drivers/usb/serial/ftdi_sio.c: read_latency_timer
    drivers/usb/serial/ftdi_sio.c: write_latency_timer: setting latency timer = 1
    drivers/usb/serial/ftdi_sio.c: create_sysfs_attrs
    drivers/usb/serial/ftdi_sio.c: sysfs attributes for FT-X
    usb 2-1: FTDI USB Serial Device converter now attached to ttyUSB0

Signed-off-by: Jim Paris <jim@jtan.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>