mfd: rtsx: Read vendor setting from config space

Normally OEMs will set vendor setting to the config space of Realtek
card reader in BIOS stage. This patch reads the setting at the first,
and configure the internal registers according to it, to improve card
reader's compatibility condition.

Signed-off-by: Wei WANG <wei_wang@realsil.com.cn>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>