cmd64x: don't clear the other channel's interrupt

Make sure to not clear the other IDE channel's interrupt when clearing an IDE
interrupt via the MRDMODE register.

Thanks to Bart for finding a coding mistake.

Bart:

This fixes regression from commit 66602c83
("cmd64x: use interrupt status from MRDMODE register (take 2)").

Extra thanks to Martin for reporting and bisecting the issue.

From: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Tested-by: Martin Rogge <marogge@onlinehome.de>
Tested-by: Milan Kocian <milon@wq.cz>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>

drivers/ide/pci/cmd64x.c

 /*
- * linux/drivers/ide/pci/cmd64x.c		Version 1.50	May 10, 2007
+ * linux/drivers/ide/pci/cmd64x.c		Version 1.51	Nov 8, 2007
  *
  * cmd64x.c: Enable interrupts at initialization time on Ultra/PCI machines.
  *           Due to massive hardware bugs, UltraDMA is only supported
@@ -339,7 +339,8 @@ static int cmd648_ide_dma_end (ide_drive_t *drive)
 	u8  mrdmode		= inb(hwif->dma_master + 0x01);
 
 	/* clear the interrupt bit */
-	outb(mrdmode | irq_mask, hwif->dma_master + 0x01);
+	outb((mrdmode & ~(MRDMODE_INTR_CH0 | MRDMODE_INTR_CH1)) | irq_mask,
+	     hwif->dma_master + 0x01);
 
 	return err;
 }