The current setting reflects about 86 Ohms of source-impedance
on the SDIO signals where the WiFi board is hooked up. PCB traces are
routed with 50 Ohms impedance and there are no serial resistors on
those traces.

This commit changes the source-impedance to 52 Ohms to better match our
hardware design.

The impedances given in this commit message refer to 3.3V operation.
Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>