flash/nor/rp2040: remove new line from error message

Change-Id: Idf3bce842b4507c1f12692b5fbcd6730637de9db
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7216
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Jonathan Bell <jonathan@raspberrypi.com>
Tested-by: jenkins