Make sbsh(4) MPSAFE: - Add a mutex to the softc and use it to protect the softc and device   hardware. - Setup interrupt handler after ether_ifattach(). - Remove unused sbsh_watchdog() routine. - Protect against concurrent attempts to load firmware.
Make sbsh(4) MPSAFE:
- Add a mutex to the softc and use it to protect the softc and device
  hardware.
- Setup interrupt handler after ether_ifattach().
- Remove unused sbsh_watchdog() routine.
- Protect against concurrent attempts to load firmware.

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