staging: dgnc: take a lock when storing value in dgnc_poll_tick

Reads of dgnc_poll_tick are protected by dgnc_poll_lock spinlock, but the write
to dgnc_poll_tick is not. It could theoretically race.

Signed-off-by: Salah Triki <salah.triki@acm.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>