soc/intel/tigerlake: Set default USB3 de-emphasis to -3.5dB

The HSIO tuning guide recommendation for the default USB3 settings is to
have de-emphasis set to -3.5dB with the equation 20*log(X/64).  0x29 results
in a value close to -3.5dB and it is the value that was used for the default
on past platforms so I used it here as well.

BUG=b:160721468
TEST=Ensure WWAN device does not disconnect during use.

Change-Id: Ia594996cb55523dacce0d4bef98cc217321c62de
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43831
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/soc/intel/tigerlake/include/soc/usb.h

@@ -132,7 +132,7 @@ struct usb3_port_config {
 #define USB3_PORT_DEFAULT(pin) { \
 	.enable           = 1, \
 	.ocpin            = (pin), \
-	.tx_de_emp        = 0x0, \
+	.tx_de_emp        = 0x29, \
 	.tx_downscale_amp = 0x00, \
 }