usb: gadget: define free_ep_req as universal function

commit 079fe5a6da616891cca1a26e803e1df2a87e9ae5 upstream.

This function is shared between gadget functions, so this avoid unnecessary
duplicated code and potentially avoid memory leaks.

Reviewed-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>