USB: gadget: ci13xxx_udc: Avoid NULL pointer dereference.

There are some instances where acquire spin_lock using mEp->lock
as one of the argument without checking availability of mEp. Since
mEp is container_of "ep", input parameter 'ep' could be null this leads
to null pointer dereference. Fix this by adding proper check before
acquiring the spin_lock.

CRs-Fixed: 2053686
Change-Id: I712343915da1f838dae6930034ad959b1f3ad4cd
Signed-off-by: Arumuga Durai A <cadurai@codeaurora.org>