staging: emxx_udc: Add comment for spinlock_t definition.

Members of nbu2ss_udc structure can change device state, maintain
completion state and control driver. Also provide access to read and
write to register. Hence, exclusive access to nbu2ss_udc is required.
The lock variable of type spinlock_t guarantees the exclusive access
and protects it.

In this patch, comment is added for spinlock_t definition, to fix the
following checkpatch issue:

CHECK: spinlock_t definition without comment

Signed-off-by: Varsha Rao <rvarsha016@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>