spi/bcm63xx: simplify bcm63xx_spi_check_transfer

bcm63xx_spi_check_transfer is only called from one place that has
t always set, so directly check the transfer's bits_per_word.

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