rtc: cmos: Use predefined value for RTC IRQ on legacy x86

When legacy devices are present on x86 machine, the RTC IRQ has
a dedicated pre-defined value. Use it instead of hard coded number.

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20200123131437.28157-2-andriy.shevchenko@linux.intel.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-cmos.c

@@ -1305,7 +1305,7 @@ static int cmos_pnp_probe(struct pnp_dev *pnp, const struct pnp_device_id *id)
 		 * hardcode it on systems with a legacy PIC.
 		 */
 		if (nr_legacy_irqs())
-			irq = 8;
+			irq = RTC_IRQ;
 #endif
 		return cmos_do_probe(&pnp->dev,
 				pnp_get_resource(pnp, IORESOURCE_IO, 0), irq);