usb: gadget: f_phonet: eliminate abuse of ep->driver data

Since ep->driver_data is not used for endpoint claiming, neither for
enabled/disabled state storing, we can reduce number of places where
we read or modify it's value, as now it has no particular meaning for
function or framework logic.

In case of f_phonet we only need to store in ep->driver_data pointer to
struct f_phonet, as it's used in pn_tx_complete() and pn_rx_complete()
callbacks. All other uses of ep->driver_data are now meaningless and can
be safely removed.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>