powerpc/powermac: Cleaning up missing null-terminate in conjunction with strncpy

Replacing strncpy with strlcpy to avoid strings that lacks null terminate.
And removed unnecessary magic numbers.
Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/powermac/bootx_init.c

@@ -246,7 +246,7 @@ static void __init bootx_scan_dt_build_strings(unsigned long base,
 		DBG(" detected display ! adding properties names !\n");
 		bootx_dt_add_string("linux,boot-display", mem_end);
 		bootx_dt_add_string("linux,opened", mem_end);
-		strncpy(bootx_disp_path, namep, 255);
+		strlcpy(bootx_disp_path, namep, sizeof(bootx_disp_path));
 	}
 
 	/* get and store all property names */