spi: topcliff-pch: Use core message validation

The topcliff-pch driver requires TX and RX buffers on all transfers, open
coding checks for this. Remove those open coded checks and instead rely on
the core functionality, which has the added bonus that it will fix up any
transfers submitted by drivers as needed rather than erroring out.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220615174138.4060912-1-broonie@kernel.org