USB: ehci-tegra: add clk_prepare/clk_unprepare

Use clk_prepare/clk_unprepare as required by the generic clk framework.

Cc: stern@rowland.harvard.edu
Cc: gregkh@linuxfoundation.org
Cc: linux-usb@vger.kernel.org
Signed-off-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>