usb: phy: omap_usb_phy: Fix USB3_PHY DPLL configuration

The index returned by get_sys_clk_index() is not exactly what we expect.
Let's not rely on that and use get_sys_clk_freq() instead.

This fixes missing USB3 devices in the Linux kernel when USB is started
in u-boot. It still doesn't fix missing USB3 devices in u-boot though.

Signed-off-by: Roger Quadros <rogerq@ti.com>