usb: gadget: Unclaim endpoints upon disconnect

HSIC data driver is not setting endpoint driver data to NULL
upon disconnect. This causes endpoints to remain claimed at
the time of function driver unbind. Endpoints will remain
unusable when driver bind happens later. This can result in
function bind failure due to lack of available(unclaimed)
endpoints if there is a series of driver unbind and bind
happens. Hence after disabling endpoint set driver data of
the endpoint to NULL upon disconnect.

CRs-Fixed: 449814
Change-Id: I8656e15ac26dfc6f4f6679b7ee2bba0bf935f0d6
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>