tcl at91samdXX.cfg: partially revert change 3722

Commit 25d7ba19c9 introduced a problem
with 'reset halt' due to setting srst_pulls_trst:

Error: cortex_m.c:595 cortex_m_halt(): can't request a halt while
  in reset if nSRST pulls nTRST

Sorry, I don't know why I overlooked it when I tested #3722.

Change-Id: I41e9473dd91a86d93cf3e78b1fbbdfe1dd188d83
Reported-by: Ladislav Laska <laska@kam.mff.cuni.cz>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3942
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

tcl/target/at91samdXX.cfg

@@ -50,7 +50,8 @@ $_TARGETNAME configure -event reset-deassert-post {
 }
 
 # SRST (wired to RESET_N) resets debug circuitry
-reset_config srst_gates_jtag srst_pulls_trst
+# srst_pulls_trst is not configured here to avoid an error raised in reset halt
+reset_config srst_gates_jtag
 
 # Do not use a reset button with other SWD adapter than Atmel's EDBG.
 # DSU usually locks MCU in reset state until you issue a reset command