bridge-stp: Make sure kernel knows about the STP state.

bridge-stp: Make sure kernel knows about the STP state.

From email:
  Yes, but it have issues:
  1) After log ERROR script should return failure (!=0) - after all, user-space
     stp was not setup correctly and kernel should know it. So "exit 1" should be
     inserted in two places:
      after log ERROR "mstpd is not running. STP might not work."
     and
      after log ERROR "Unknown action given: ${action}."
  2) If zone does not exist script also should return failure (!=0). Kernel
     should know that user-space stp was not started.
  3) I assume your "assert " function returns failure(non-zero) too ;)
  4) mstpctl MUST be run through exec call.
  5) thanks to above "assert" in the start case can be omitted - execution flow
     will never go that far.
  6) The final "exit 0" is superfluous.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>