scsi: sd: Cleaned up comment references to @sdp argument explanation.

In sd.c there are two comment references to 'struct scsi_device *sdp' as
an argument.  One of the references has a typo and the other should be a
reference to 'struct device *dev' instead.

Fixed by correcting the typo in the first and changing the explanation
in the second.

Signed-off-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>