[SCSI] megaraid_mbox: remove scsi_assign_lock usage

also remove the adapter->host_lock alias for adapter->lock and remove
some superflous locking aswell as removing the tiny locking wrappers
for the EH routines.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>