usb: phy: tegra: Use DT helpers for phy_type

Use the new of_usb_get_phy_mode helper function for parsing phy_type
from the device tree.

Signed-off-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>