scsi: g_NCR5380: Use unambiguous terminology for PDMA send and receive

The word "read" may be used to mean "DMA read operation" or "SCSI READ
command", though a READ command implies writing to memory.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>