spi/bcm63xx: check spi bits_per_word in spi_setup

Instead of fixing up the bits_per_word (which the spi subsystem already
does for us), check it for supported values.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>