ci13xxx_udc: Acquire spinlock before calling ep_nuke()

The commit b0ad433 (USB: ci13xxx_udc: Fix kernel panic during
composition switch) adds ep_nuke() function in gadget_stop_activity().

_ep_nuke() releases and re-acquires spinlock on every entry of the
function. Acquiring spinlock before calling _ep_nuke() is necessary,
else it may end up in unlocking the lock twice, as it is already
unlocked before calling _ep_nuke(), which may lead to crash.

Hence, acquire spinlocks before calling _ep_nuke() function.

CRs-Fixed: 486502
Change-Id: Ia80b9facb8e592f8fb41879dc08f9f1a04a20b48
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>