r241438 broke IPMI access on Sun Fire X2200 M2(BCM5715).
Fix the IPMI regression by sending BGE_FW_DRV_STATE_UNLOAD to
ASF/IPMI firmware in driver attach phase.  Sending heartheat to
ASF/IPMI is enabled only after upping interface so
setting driver state to BGE_FW_DRV_STATE_START in attach phase
broke IPMI access.
While I'm here, add NVRAM arbitration lock before performing
controller reset. ASF/IPMI firmware may be able to access the NVRAM
while controller reset is in progress.  Without the arbitration
lock before resetting the controller, ASF/IPMI may not initialize
properly.

Special thanks to Miroslav Lachman who provided full remote
debugging environments.