scsi: megaraid_sas: Expose fw_cmds_outstanding through sysfs

Expose FW outstanding commands (fw_outstanding) through sysfs interface.
This helps in debugging certain performance issues in the field.

Signed-off-by: Sasikumar Chandrasekaran <sasikumar.pc@broadcom.com>
Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>