usb: gadget: mv_udc: fix a clerical error

The max size of data payload is in bit0 - bit10, so we need use 0x7ff
as the bitmask to fetch from usb_endpoint_descriptor.wMaxPacketSize.

Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>