spi/bcm63xx: don't substract prepend length from total length

The spi command must include the full message length including any
prepended writes, else transfers larger than 256 bytes will be
incomplete.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org