spi: lpspi: Add the missing NULL check

The spi_transfer *t will be used in one transfer whatever. If t is NULL,
there has no need to try sending data, so add an error return here.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Acked-by: Fugang Duan <Fugang.duan@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>