Make messages reference "DAP" if they're actually transport-agnostic, or
"JTAG-DP" when they're JTAG-specific.  Saying SWJ-DP is often wrong (on
most Cortex-A8 chips) and is confusing even if correct (since we don't
yet support SWD).

Rename a JTAG-specific routine to jtagdp_transaction_endcheck() to highlight
that it's JTAG-specific, and that identify DAP clients undesirably depending
on JTAG.  (They will all need to change for SWD support.)

Shrink a few overlong lines of code.  Copy a comment from code removed
in a previous patch (for the ARMv7-M "dap baseaddr" command).

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