usb: core: Limit HCD old enumeration scheme request to root ports

HCD_OLD_ENUM HCD flag is introduced to use old (or short) enumeration
scheme for the devices attached on the corresponding USB bus. The old
enumeration scheme is used by the MSM HSIC controller to avoid multiple
reset during enumeration. This is useful when a known device is attached
to the USB bus, which is the case with MSM HSIC. When a HSIC HUB is
connected, any device can be connected on the HSIC USB bus. The new
enumeration scheme supports wide variety of USB devices. Hence limit
the old enumeration scheme request of a HCD to only root ports.

Change-Id: I6035c2a766787819eb8d0e1fa774e0c79d6e958b
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>