jtag: only change state if necessary

All the other drivers will only change the state if required.
This brings all the other drivers inline with this behaviour.

The original issue relates to problems on xscale commit 7989000e0969c1ccf69acbc3ce649a020bc1ee66

Change-Id: Ifc90ec2eef68a70a14f37c00931a07982bfa200c
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1114
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>