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

commit 86b3bde003e6bf60ccb9c09b4115b8a2f533974c upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>