usb: dwc3: omap: add dwc3_omap_set_utmi_mode() function

Move find and set the utmi mode to its own separate function.
Improve code readability, decrease the dwc3_probe() size.

Signed-off-by: George Cherian <george.cherian@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>