usb: gadget: unconfigure ep during disable

The dwc3-msm function msm_ep_configure is called each
time the USB is connected. In order to allow this to
happen more than once, the corresponding msm_ep_unconfigure
function has to be called during the function disable.
Without the fix, the call to msm_ep_configure returns an error
which leads to the composition switch to fail. In turn, this
causes a call to reset_config which calls gbam_disconnect
during the ongoing process of gbam2bam_connect_work. The
disconnect function nullifies a pointer that the connect function
uses - thereby causing a null pointer dereferencing

Change-Id: I0cf567f7d91cadd490ee7bf88e67bd8ad3e6dfa7
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>