[SCSI] megaraid (legacy): remove scsi_assign_lock usage

just take the adapter lock in megaraid_queue.  Additional benefit is
that we can get rid of the awkward conditional locking in
mega_internal_command.

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