flash/nor/rp2040: check target halted before flash operation

flash/nor/rp2040: check target halted before flash operation

Flash read_id/erase/write operation on running target failed
in target_run_algorithm() anyway. It generated lot of error messages.

Check the target state and bail out early if target is running.

Change-Id: I903f5f38c8e61016e5002b235e5f07803bd2ec4e
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7215
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Jonathan Bell <jonathan@raspberrypi.com>