phy: tegra: mark tegra_xusb_lane_lookup_function() static

We get 1 warning when building kernel with W=1:
drivers/phy/tegra/xusb.c:104:5: warning: no previous prototype for 'tegra_xusb_lane_lookup_function' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
So this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>