s390/sclp: fix declaration of _sclp_print_early()

_sclp_print_early() has return value: at present, return 0 for OK, 1 for
failure. It returns '%r2', so use 'long' as return value (upper caller
can check '%r2' directly). The related warning:

    CC      arch/s390/boot/compressed/misc.o
  arch/s390/boot/compressed/misc.c:66:8: warning: type defaults to 'int' in declaration of '_sclp_print_early' [-Wimplicit-int]
   extern _sclp_print_early(const char *);
          ^

At present, _sclp_print_early() is only used by puts(), so can still
remain its declaration in 'misc.c' file.

[heiko.carstens@de.ibm.com]: move declaration to sclp.h header file

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>