usb: gadget: function: phonet: balance usb_ep_disable calls

f_phonet's ->set_alt() method will call usb_ep_disable()
potentially on an endpoint which is already disabled. That's
something the gadget/function driver must guarantee that it's
always balanced.

In order to balance the calls, just make sure the endpoint
was enabled before by means of checking the validity of
driver_data.

Reported-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>