tigertool: fix regex

Change $ to \s in regex.

BUG=None (tigertail serserialno fail
TEST=./tigertool.py --setserialno test
BRANCH=none

Change-Id: I006ff8e70b3d20be44590c2050f9e341f10a2d53
Signed-off-by: Nick Sanders <nsanders@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2008450
Reviewed-by: YH Lin <yueherngl@chromium.org>
Reviewed-by: Aseda Aboagye <aaboagye@chromium.org>

extra/tigertool/ecusb/tiny_servo_common.py

@@ -115,7 +115,7 @@ def do_serialno(serialno, pty):
     ptyError: on command interface error.
   """
   cmd = 'serialno set %s' % serialno
-  regex = 'Serial number: (.*)$'
+  regex = 'Serial number:\s+(\S+)'
 
   results = pty._issue_cmd_get_results(cmd, [regex])[0]
   sn = results[1].strip().strip('\n\r')