usb: phy: omap-usb3: fix dpll clock index

Correction of the omap_usb3_dpll_params array when the sys_clk_rate is
20MHz.

Signed-off-by: Nikhil Devshatwar <nikhil.nd@ti.com>
Signed-off-by: Ruchika Kharwar <ruchika@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>