spi/bcm63xx: inline bcm63xx_spi_check_transfer

It only does one check, so just do the check directly in the caller.

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