spi/bcm63xx: Remove unused variable

This fixes the following warning:

drivers/spi/spi-bcm63xx.c: In function 'bcm63xx_spi_setup':
drivers/spi/spi-bcm63xx.c:157:6: warning: unused variable 'ret'

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>