spi: core: Fix transfer failure when master->transfer_one returns positive value

spi: core: Fix transfer failure when master->transfer_one returns positive value

master->transfer_one returns positive value is not a error.
So set ret to 0 when master->transfer_one returns positive value.
Otherwise, I hit "spi_master spi0: failed to transfer one message from queue"
error when my transfer_one callback returns 1.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>