target: fix copy&paste error in cget -coreid

target: fix copy&paste error in cget -coreid

Command should return target->coreid, not target->working_area_size
Solution appeared as an off topic fix in http://openocd.zylin.com/4935

Change-Id: Ic9701ce2001c75060dbaa75fbd013ed475a352e5
Suggested-by: Graham Sanderson <graham.sanderson@gmail.com>
Reported-by: Tim Newsome <tim@sifive.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5035
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>