jtag_add_dr/ir_scan() now takes the tap as the first
argument, rather than for each of the fields passed
in.

The code never exercised the path where there was
more than one tap being scanned, who knows if it even
worked.

This simplifies the implementation and reduces clutter
in the calling code.

use jtag_add_ir/dr_plain_scan() for more fancy situations.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>