usb: gadget: legacy: tcm: 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 tcm, ep->driver_data was used only for endpoint claiming so
we can simplify code by reducing it. We also remove give_back_ep()
function which is not needed after all - when error code is returned
from bind() function, composite will release all endpoints anyway.

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