usb: phy: use kzalloc to allocate struct tegra_usb_phy

Use kzalloc instead of kmalloc to allocate struct tegra_usb_phy.
This ensures that all function pointers in member u_phy are
initialized to NULL.

Signed-off-by: Venu Byravarasu <vbyravarasu@nvidia.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>