MFC 248705,253812,253813:

• Unlock IPMI sc while performing requests via KCS and SMIC interfaces.
• empirical testing showed that 3 seconds is just too slow for GET_DEVICE_ID to return on newer Dell hardware. Bump to 6 second timeouts until someone has a better idea on how to handle this
• Check for ipmi_attached in ipmi_isa_probe as a suggested alternative to ipmi_isa_attach. This keeps unintended but harmless noise about "ipmi1" from appearing in the boot up sequence.