tcl/interface/ftdi/sheevaplug: Fix FTDI channel configuration

tcl/interface/ftdi/sheevaplug: Fix FTDI channel configuration

The migration from the old ft2232 driver to the new generic ftdi driver
ended up breaking support for the SheevaPlug device. The old driver
defaulted to channel 1, but numbered the channels 1 to 4. The new driver
starts at 0. The SheevaPlug JTAG is on interface A (interface B is the
serial console), so it should be using channel 0. Fix this. Confirmed
as working; serial console remains available and a new u-boot image can
be transferred across using the JTAG link.

See also Debian Bug#837989; https://bugs.debian.org/837989

Change-Id: I4ac2bfeb0d1e7e99d70fa47dc55f186e6af2c542
Signed-off-by: Jonathan McDowell <noodles@earth.li>
Reviewed-on: http://openocd.zylin.com/4206
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins