phy: tegra: xusb: Fix return value of tegra_xusb_find_port_node function

callers of tegra_xusb_find_port_node() function only do NULL checking for
the return value. return NULL instead of ERR_PTR(-ENOMEM) to keep
consistent.

Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20211213020507.1458-1-linmq006@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>
