usb: hub: Add a new API to test if a hub device is root hub

Sometimes we need know if a given hub device is root hub or not.
Add a new API to test this. This removes the xHCI driver's own
version is_root_hub() and change to use the new API.

While we are here, remove the unused/commented out get_usb_device()
in the xHCI driver too.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
(cherry picked from commit 46c1d49330fe21b3f9d2f7577ee4268248e7b666)
Signed-off-by: Santan Kumar <santank@codeaurora.org>

Change-Id: Ia27ee3b541f936e3eebf6ecb57c3499c08da3781
3 files changed