usb: mv_udc: Implement better qTD item accessor

The code for retrieving qTD item for particular endpoint is hard
to understand, moreover it's duplicated all over the driver. Move
the code into single nice and documented function.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Lei Wen <leiwen@marvell.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>