spi: allow registering empty spi_board_info lists

Many boards form list of spi_board_info entries depending on config,
and it is possible to end up with empty list. Do not report error
in such cases.
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -690,7 +690,7 @@ int spi_register_board_info(struct spi_board_info const *info, unsigned n)
 	int i;
 
 	if (!n)
-		return -EINVAL;
+		return 0;
 
 	bi = kcalloc(n, sizeof(*bi), GFP_KERNEL);
 	if (!bi)