scsi: megaraid_sas: Update function description

Function parameter 'pdev 'is described as Generic Device Structure. It is a
PCI device structure.

Link: https://lore.kernel.org/r/20201102164730.324035-4-vaibhavgupta40@gmail.com


Signed-off-by: Vaibhav Gupta <vaibhavgupta40@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>