s390/sclp: avoid using strncmp with hardcoded length

"earlyprintk" option documentation does not clearly state which
platform supports which additional values (e.g. ",keep"). Preserve old
option behaviour and reuse str_has_prefix instead of strncmp for prefix
testing.
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/early_printk.c

@@ -25,7 +25,7 @@ static int __init setup_early_printk(char *buf)
 	if (early_console)
 		return 0;
 	/* Accept only "earlyprintk" and "earlyprintk=sclp" */
-	if (buf && strncmp(buf, "sclp", 4))
+	if (buf && !str_has_prefix(buf, "sclp"))
 		return 0;
 	if (!sclp.has_linemode && !sclp.has_vt220)
 		return 0;