soc/mediatek: Fix USB enumeration issue

Some USB 3.0 devices fail to be enumerated after USB reset, and xhci
port status register shows the device is disconnected. After measuring
the USB signal, we found that the USB disconnect threshold was lower and
that the disconnect event was triggered unexpectedly.

USB designers suggest changing discth to 15.

BUG=b:122047652
TEST=emerge-kukui coreboot chromeos-bootimage

Change-Id: I0e8556035b49d693a42cbe1099a6882a1c0ed0d1
Signed-off-by: Changqi Hu <changqi.hu@mediatek.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35150
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>

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