scsi: mpt3sas: function pointers of request descriptor

This code refactoring introduces function pointers.

Host uses Request Descriptors of different types for posting an entry onto
a request queue. Based on controller type and capabilities, host can also
use atomic descriptors other than normal descriptors.  Using function
pointer will avoid if-else statements

Signed-off-by: Suganath Prabu S <suganath-prabu.subramani@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>