spi: remove unused variable in tegra_slink_read_rx_fifo_to_client_rxbuf()

The variable bits_per_word is initialized but never used
otherwise, so remove the unused variable.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-By: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>