jtag: core: report TRST and SRST as asserted only when they really are

When SRST is never enabled, the global jtag_srst variable is left at its
initial state, that is, -1, and it does _not_ mean SRST is currently
asserted. Same about TRST.

Fixes "reset halt" in cases when srst_pulls_trst but srst usage is not
enabled.

Change-Id: I8d2e9120479de4cfbf5561033926c9ef945eecc9
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3943


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>