target/arm_dap: clean up dap_configure code

dap_configure() contained first time init related tasks, as the call to
dap_init_instance() and the check for configured tap.

Move all first time init related stuff to dap_create() to make dap_configure()
usable in eventual stand-alone 'dap configure' command.

Change-Id: Ia86eadb4e960ce54e8581630d01af75720d2318d
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6702
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>