usb: gadget: remove ecm_cleanup in disconnect

ecm_cleanup should be called in unbind since ecm_ipa_init/configure
are called at bind, and this was indeed changed lately.
This instance causes a crash since ecm_upa_cleanup is called before
ecm_ipa_disconnect is called. (since it's called in the disconnect workQ)

Change-Id: I0ec187b89c147ae489ce9c470f6401d58fe58f91
Signed-off-by: Lena Salman <esalman@codeaurora.org>
Signed-off-by: Amit Blay <ablay@codeaurora.org>