fix(utime.c): Fix time zone offset handling in time_ms

Another bug in utime.c, this time in utime.time_ms():

The time zone offset is added with the wrong sign, as a result of wrong precedence when adding parentheses to the multiplication with 1000:

Testcase:

import utime

s = utime.time()
ms = utime.time_ms()
diff = s - ms // 1000

if diff != 0:
  print("time difference:", diff)
  # will print 14400, or 4 hours, with the old code
  # should print 0