net: make netpoll_rx return bool for !CONFIG_NETPOLL

"netpoll: Use 'bool' for netpoll_rx() return type." missed the case when
CONFIG_NETPOLL is disabled.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>