ARM: dts: imx6sx-sdb: update TX D_CAL for USBPHY

We need to change trimming value (as a percentage) of the 17.78mA TX
reference current for better signal quality. With this change, we
can pass the eye-diagram test on this board.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>