mpt2sas: delay scsi_add_host call to work with scsi-mq

In _scsih_probe, delay the call to scsi_add_host until the host has been
fully set up.

Otherwise, the default .can_queue value of 1 causes scsi-mq to set the block
layer request queue size to its minimum size, resulting in awful performance.

In _scsih_probe error handling, call mpt3sas_base_detach rather than
scsi_remove_host to properly clean up in reverse order.

In _scsih_remove, call scsi_remove_host earlier to clean up in reverse order.

Signed-off-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nagalakshmi Nandigama <Nagalakshmi.Nandigama@avagotech.com>
Signed-off-by: Sreekanth Reddy <Sreekanth.Reddy@avagotech.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>