ft2232: add a mechanism to specify channel in layout structs

FT2232-family chips have two or more MPSSE modules.   FTDI documentation
calls these channels.  JTAG adapter drivers thus need to be able to choose
which channel to use.  (For example, one channel may connect to a board's
microcontroller, while another connects to a CPLD.)

Since each channel has its own USB interface, libftdi (somewhat confusingly)
identifies channels using INTERFACE_* symbols.  Most boards use INTERFACE_A
for JTAG, which is the default in OpenOCD.  But some wire up a different one.

Note that there are two facets of what makes a wiring "layout":

 - The mapping between debug signals map and channel signals ... embedded
   in C functions.

 - Label used in Tcl configuration scripts ... part of the "layout" structure.

By letting the channel be part of the layout struct, we permit sharing the C
functions between Tcl-visible layouts, when those signal mappings are reused.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

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