timers/x86/hpet: Use HPET_COUNTER to specify the hpet counter in vread_hpet()

vread_hpet() uses "0xf0" as the offset of the hpet counter. To
clarify the meaning of this code, it should use symbolic name,
HPET_COUNTER, instead.
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/vdso/vclock_gettime.c

@@ -60,7 +60,7 @@ notrace static cycle_t vread_tsc(void)
 
 static notrace cycle_t vread_hpet(void)
 {
-	return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + 0xf0);
+	return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + HPET_COUNTER);
 }
 
 #ifdef CONFIG_PARAVIRT_CLOCK