spi/bcm63xx: don't reject reads >= 256 bytes

The rx_tail register is only 8 bit wide, so it will wrap around
after 256 read bytes. This makes it rather meaningless, so drop any
usage of it to not treat reads over 256 as failed.

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