spi/bcm63xx: remove unused variable bs from bcm63xx_spi_setup

It is only written, but never read.

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