mmc: rtsx: Configure SD_CFG2 register in sd_rw_multi

For Realtek card reader, internal regsiter SD_CFG2 should be configured
before transferring data.
The default value of SD_CFG2 is proper for writing data. But for reading
sequence, the timing is not good enough. So in some extreme circumstance,
card reader may sample the response data from the card as good even if
the data is wrong. And this will cause the bad consequence.
In the prior version, the value of this register has been calculated,
but forgotten to write back to the internal register.

Signed-off-by: Wei WANG <wei_wang@realsil.com.cn>
Acked-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>