spi/bcm63xx: remove unused rx_tail variable

Fixes the following warning:
drivers/spi/spi-bcm63xx.c:125:5: warning: unused variable 'rx_tail' [-Wunused-variable]
  u8 rx_tail;
     ^

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Mark Brown <broonie@kernel.org>