spi: oc-tiny: Simplify tiny_spi_txrx_bufs implementation when irq is not used

Currently we have similar code for (txp && rxp), (txp && !rxp), (!rxp & txp),
and (!txp && !rxp) cases. This patch refactors the code a bit to avoid
duplicate similar code.

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