spi/pxa2xx: Consider CE4100's FIFO depth

For PXA the default threshold is FIFO_DEPTH / 2. Adjust this value for
CE4100.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>