target: simplify register get/set ops

target: simplify register get/set ops

No need to indirect from registered integers to pointers.
Just stash the pointers directly in the register struct,
and don't even bother registering.

This is a small code shrink, speeds register access just
a smidgeon, and gets rid of another rude exit() path.

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