s390/sclp: convert early sclp console code to C

The 31-bit assembler code for the early sclp console is error
prone as git commit fde24b54d976cc123506695c17db01438a11b673
"s390/sclp: clear upper register halves in _sclp_print_early"
has shown.

Convert the assembler code to C.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/include/asm/sclp.h b/arch/s390/include/asm/sclp.h
index f6ff060..821dde5 100644
--- a/arch/s390/include/asm/sclp.h
+++ b/arch/s390/include/asm/sclp.h
@@ -79,6 +79,6 @@
 int sclp_pci_deconfigure(u32 fid);
 int memcpy_hsa(void *dest, unsigned long src, size_t count, int mode);
 void sclp_early_detect(void);
-long _sclp_print_early(const char *);
+int _sclp_print_early(const char *);
 
 #endif /* _ASM_S390_SCLP_H */