FT2232: lookup and save layout just once

FT2232: lookup and save layout just once

Streamline use of the layout:  have the "ft2232_layout" command
look it up and save the result, instead of having a few different
chunks of code looking it up later, and saving just its name (which
is already part of the layout).  This

   - is cleaner
   - reports errors sooner
   - facilitates earlier adapter-specific setup
   - removes unused "default to "usbjtag" logic

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