spi: npcm-fiu: remove set but not used variable 'retlen'

drivers/spi/spi-npcm-fiu.c: In function npcm_fiu_read:
drivers/spi/spi-npcm-fiu.c:472:9: warning:
 variable retlen set but not used [-Wunused-but-set-variable]

It is never used, so remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20190905072436.23932-1-yuehaibing@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>