mtd: uboot: Add meaningful error message

The current error message in get_part if CONFIG_MTDPARTS is disabled is
"offset is not a number" which is confusing and doesn't help at all.

Change that for something that might give a hint on what's going on.
Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: NTom Rini <trini@konsulko.com>

drivers/mtd/mtd_uboot.c

@@ -37,7 +37,7 @@ static int get_part(const char *partname, int *idx, loff_t *off, loff_t *size,
 
 	return 0;
 #else
-	puts("offset is not a number\n");
+	puts("mtdparts support missing.\n");
 	return -1;
 #endif
 }