scsi: mpt3sas: Improve kernel-doc headers

Avoids that warnings about the kernel headers appear when building with
W=1. Remove useless "@Returns - Nothing" clauses. Change "@Return - " into
"Return: ".

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Sathya Prakash <sathya.prakash@broadcom.com>
Cc: Chaitra P B <chaitra.basappa@broadcom.com>
Cc: Suganath Prabu Subramani <suganath-prabu.subramani@broadcom.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>