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

usb: gadget: f_midi: 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_midi we only need to store in ep->driver_data pointer to
struct f_midi, as it's used in f_midi_complete() callback and related
functions. 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>