staging: emxx_udc: add missing usb_ep_set_maxpacket_limit()

Since maxpacket_limit was introduced all UDC drivers should use
usb_ep_set_maxpacket_limit() function instead of setting maxpacket value
manually. ep.maxpacket_limit contains actual maximum maxpacket value
supported by hardware which is needed by epautoconf.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>