usb: gadget: qc_ecm: allow for calling set_alt(1) twice

As part of the disconnect sequence, unconfigures the msm ep in case
that set_alt(1) is called. This is requires in order to allow the
subsequent calls to msm_ep_config() in the function.

CRs-fixed: 733358
Change-Id: I725ec0a040bcb6dce2fdafb0467035c75179ff17
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>