phy: sun4i-usb: Remove .disc_thresh where not applicable

.disc_thresh is not used when .type = sun50i_h6_phy, so providing a
value for it is misleading.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Link: https://lore.kernel.org/r/20220203013558.11490-2-samuel@sholland.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>