usb: gadget: move find_ep() from epautoconf to udc-core

Move find_ep() to udc-core and rename it to gadget_find_ep_by_name().
It can be used in UDC drivers, especially in 'match_ep' callback after
moving chip-specific endpoint matching logic from epautoconf to UDC
drivers.

Replace all calls of find_ep() function with gadget_find_ep_by_name().

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