[SCSI] mpt fusion: add use of shost_priv and remove all the typecasting

The driver is currently typecasting to obtain the shost hostdata. The
driver is updated to use the shost_priv macro.

Signed-off-by: Eric Moore <Eric.Moore@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>