usb: gadget: bdc: use readl_poll_timeout() to simplify code

Use readl_poll_timeout() to poll register status

Cc: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>