mpc8xxx_spi: put max_cs to use

Currently, max_cs is write-only; it's just set in
mpc8xxx_spi_ofdata_to_platdata and not otherwise used.

My mpc8309 was always resetting during an "sf probe 0". It turns out
dm_gpio_set_dir_flags() was being called with garbage, since nothing
had initialized priv->gpios[0] - our device tree used "cs-gpios"
rather than "gpios", so gpio_request_list_by_name() had returned 0.

That would have been a lot easier to figure out if the chip select
index was sanity checked, so rename max_cs to cs_count, and reject a
xfer with a too large cs index.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>