mpt2sas: Use mpi headers from mpt3sas

Use a single set of the hardware description headers instead of having
them in the source tree twice.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sreekanth Reddy <sreekanth.reddy@avagotech.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>