drivers: spi: spi_mcux_ecspi: Ignore chip select channel with cs-gpios

The internal chip selects are limited to 4, however when using GPIOS
does not pose this limitation.
Also set internal channel to 0 if GPIOS are used.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>
(cherry picked from commit 7250e987)