spi: rockchip: precompute rx sample delay

Now that we no longer potentially change spi clock
at runtime we can precompute the rx sample delay
at probe time rather than for each transfer.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Mark Brown <broonie@kernel.org>