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: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>