s390/sclp: sign extend return value of _sclp_print_early()

_sclp_print_early() has a return value, but misses to sign extend it
if called from 64 bit code.
This is not really a bug, since currently no caller cares what the
return value is.

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