usb: mv_udc: Implement better QH accessor

The code for retrieving QH 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>