s390/cpu_mf: remove register variable in __ecctr()

Using a register variable for r4 is not necessary.  Let the
compiler decide the register to be used.
Reported-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NHendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/cpu_mf.h

@@ -174,7 +174,7 @@ static inline int lcctl(u64 ctl)
 /* Extract CPU counter */
 static inline int __ecctr(u64 ctr, u64 *content)
 {
-	register u64 _content asm("4") = 0;
+	u64 _content;
 	int cc;
 
 	asm volatile (