tegra: usb: move [start|stop]_port into ehci_hcd_[init|stop]

The ehci_hcd entry points were just calling into the Tegra USB
functions. Now that they are in the same file we can just move over the
implementation.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>