cmd64x: potential buffer overflow in cmd64x_program_timings()

The "drive->dn" value is a u8 and it is controlled by root only, but
it could be out of bounds here so let's check.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/ide/cmd64x.c

@@ -66,6 +66,9 @@ static void cmd64x_program_timings(ide_drive_t *drive, u8 mode)
 	struct ide_timing t;
 	u8 arttim = 0;
 
+	if (drive->dn >= ARRAY_SIZE(drwtim_regs))
+		return;
+
 	ide_timing_compute(drive, mode, &t, T, 0);
 
 	/*