[SCSI] mpt3sas: Added a driver module parameter max_msix_vectors

Added a driver module parameter max_msix_vectors. Using this module parameter
the maximum number of MSI-X vectors could be set.

The number of MSI-X vectors used would be the minimum of MSI-X vectors
supported by the HBA, the number of CPU cores and the value set to
max_msix_vectors module parameter.

The default value of this module parameter is set to 8. The default value of
this parameter is set to 8 inorder to reduce the amount of memory required for
Reply Descriptor Post queue.  This is because with the higher MSI-X vectors,
some times kernel is not able to allocate the requested amount of memory and
crash is observed. To overcome this problem, the default value is set to 8.

Signed-off-by: Sreekanth Reddy <Sreekanth.Reddy@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>