usb: phy: tegra: Remove unnecessary 'dev' field

struct usb_phy already has a field for the device pointer, so this
unnecessary field can be removed.

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>