spi: rockchip: handle zero length transfers without timing out

Previously zero length transfers submitted to the Rokchip SPI driver would
time out in the SPI layer. This happens because the SPI peripheral does
not trigger a transfer completion interrupt for zero length transfers.

Fix that by completing zero length transfers immediately at start of
transfer.

Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>
Link: https://lore.kernel.org/r/20210827050357.165409-1-t.schramm@manjaro.org


Signed-off-by: Mark Brown <broonie@kernel.org>