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: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>