spi/bcm63xx: remove useless call to bcm63xx_spi_check_transfer()

Calling bcm63xx_spi_check_transfer() with a NULL argument does not do
anything useful that the core spi code is not doing already, remove
this superfluous call.

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