usb: gadget: u_serial: 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 u_serial ep->driver_data stores pointer to struct gs_port,
which is referenced in many places in code. Code using ep->driver_data
to mark endpoint as enabled/disabled has been removed.

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