usb: dwc2: check return value after calling platform_get_resource()

It will cause null-ptr-deref if platform_get_resource() returns NULL,
we need check the return value.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20210831084236.1359677-1-yangyingliang@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>