s390/diag: use pfn_to_phys() instead of open coding

Use pfn_to_phys() instead of open coding to make it
clear what the code is doing.
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/diag.h

@@ -47,8 +47,8 @@ static inline void diag10_range(unsigned long start_pfn, unsigned long num_pfn)
 {
 	unsigned long start_addr, end_addr;
 
-	start_addr = start_pfn << PAGE_SHIFT;
-	end_addr = (start_pfn + num_pfn - 1) << PAGE_SHIFT;
+	start_addr = pfn_to_phys(start_pfn);
+	end_addr = pfn_to_phys(start_pfn + num_pfn - 1);
 
 	diag_stat_inc(DIAG_STAT_X010);
 	asm volatile(