usb: gadget: aspeed: improve vhub port irq handling

This patch evaluates vhub ports' irq mask before going through per-port
irq handling one by one, which helps to speed up irq handling in case
there is no port interrupt.

Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>