spi: rockchip: Fix error in SPI slave pio read

The RXFLR is possible larger than rx_left in Rockchip SPI, fix it.

Fixes: 01b59ce5

 ("spi: rockchip: use irq rather than polling")
Signed-off-by: Jon Lin <jon.lin@rock-chips.com>
Tested-by: Emil Renner Berthing <kernel@esmil.dk>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Emil Renner Berthing <kernel@esmil.dk>
Link: https://lore.kernel.org/r/20200723004356.6390-3-jon.lin@rock-chips.com
Signed-off-by: Mark Brown <broonie@kernel.org>