tsnep: Add helper for RX XDP_RING_NEED_WAKEUP flag

Similar chunk of code is used in tsnep_rx_poll_zc() and
tsnep_rx_reopen_xsk() to maintain the RX XDP_RING_NEED_WAKEUP flag.
Consolidate the code to common helper function.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>