usb: gadget: mv_udc: reduce the delay interval

There are several places use udelay(LOOPS_USEC) to wait the status to be
changed, but the delay interval is a bit too long, so reduce it to
enhance the performance.

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