spi: core: make zero length transfer valid again

Zero length transfer becomes invalid since
"spi: core: Validate length of the transfers in message" commit,
but it should be valid to support an odd device, for example, which
requires long delay between chipselect and the first transfer, etc.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
(cherry picked from commit 6ea31293)
(cherry picked from commit 2e6324d5d3d41454776a6be0b369c758448242ea)