[SCSI] Add spi_populate_*_msg functions

Introduce new helpers:
 - spi_populate_width_msg()
 - spi_populate_sync_msg()
 - spi_populate_ppr_msg()

and use them in drivers which already enable the SPI transport.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

drivers/scsi/aha152x.c

@@ -1708,12 +1708,7 @@ static void seldo_run(struct Scsi_Host *shpnt)
 		ADDMSGO(BUS_DEVICE_RESET);
 	} else if (SYNCNEG==0 && SYNCHRONOUS) {
     		CURRENT_SC->SCp.phase |= syncneg;
-		ADDMSGO(EXTENDED_MESSAGE);
-		ADDMSGO(3);
-		ADDMSGO(EXTENDED_SDTR);
-		ADDMSGO(50);		/* 200ns */
-		ADDMSGO(8);		/* 8 byte req/ack offset */
-
+		MSGOLEN += spi_populate_sync_msg(&MSGO(MSGOLEN), 50, 8);
 		SYNCNEG=1;		/* negotiation in progress */
 	}