[SCSI] megaraid_sas: Optimize HostMSIxVectors setting

The following patch for megaraid_sas removes an incorrect comment and
optimizes the setting of HostMSIxVectors.  This was found during a
code review by Tomas Henzl @ RedHat.

Signed-off-by: Adam Radford <aradford@gmail.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>