usb: ulpi: make ops struct constant

None of the core ulpi functions perform any changes to the operations
struct, and logically as a struct that contains function pointers
there's no reason it shouldn't be constant.

Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Tal Shorer <tal.shorer@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>