x86: HPET: convert WARN_ON to WARN_ON_ONCE

It is possible to flood the console with call traces if the WARN_ON
condition is true because of the frequency with which this function is
called.

Signed-off-by: Matt Fleming <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Thomas Gleixner <[e-mail]>

arch/x86/kernel/hpet.c

···
 	 * what we wrote hit the chip before we compare it to the
 	 * counter.
 	 */
-	WARN_ON((u32)hpet_readl(HPET_T0_CMP) != cnt);
+	WARN_ON_ONCE((u32)hpet_readl(HPET_T0_CMP) != cnt);
 
 	return (s32)((u32)hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;
 }