spi/bcm63xx: remove spi chip select validity check

The check would belong in bcm63xx_spi_setup if the spi subsystem
weren't already doing the check for us, so just drop it.

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