s390/mm: get rid of __ASSEMBLY__ guards within pgtable.h

We have C code also outside of #ifndef __ASSEMBLY__. So these
guards seem to be quite pointless and can be removed.

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>