mmc: rtsx_pci: Fix support for speed-modes that relies on tuning

The TX/RX register should not be treated the same way to allow for better
support of tuning. Fix this by using a default initial value for TX.

Signed-off-by: Ricky Wu <ricky_wu@realtek.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200316025232.1167-1-ricky_wu@realtek.com


[Ulf: Updated changelog]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>