s390/pci: simplify __pciwb_mio() inline asm

The PCI Write Barrier instruction ignores the registers encoded in it.
There is thus no need to explicitly set the register to zero or to
associate it with a variable at all. In the resulting binary this removes
an unnecessary lghi and it makes the code simpler.
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/pci/pci_insn.c

@@ -365,10 +365,7 @@ EXPORT_SYMBOL_GPL(zpci_write_block);
 
 static inline void __pciwb_mio(void)
 {
-	unsigned long unused = 0;
-
-	asm volatile (".insn    rre,0xb9d50000,%[op],%[op]\n"
-		      : [op] "+d" (unused));
+	asm volatile (".insn    rre,0xb9d50000,0,0\n");
 }
 
 void zpci_barrier(void)