Relax polling check if not in JTAG mode

Relax polling check if not in JTAG mode

Polling was disabled based on global variables jtag_trst and jtag_srst
which were never touched in non-JTAG mode. Modify the check and remove
the ugly workaround to avoid calls to a possibly uninitialized JTAG
subsystem.

Change-Id: I3b18c81e0fba7aaf35afe6f08c3fe8fa6f8443fd
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2143
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>