[SCSI] megaraid_sas: cleanup queue command path

This patch (originally submitted by Christoph Hellwig) removes code
duplication in megasas_build_cmd.  It also defines
MEGASAS_IOC_FIRMWARE32 to allow 64 bit compiled applications to work.

Signed-off-by: Sumant Patro <Sumant.Patro@lsil.com>
Rejections fixed and
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>