net: sched: export __netdev_watchdog_up()

Since the quiesce/activate rework, __netdev_watchdog_up() is directly
called in the ucc_geth driver.

Unfortunately, this function is not available for modules and thus
ucc_geth cannot be built as a module anymore. Fix it by exporting
__netdev_watchdog_up().

Since the commit introducing the regression was backported to stable
branches, this one should ideally be as well.

Fixes: 79dde73c ("net/ethernet/freescale: rework quiesce/activate for ucc_geth")
Signed-off-by: Valentin Longchamp <valentin@longchamp.me>
Signed-off-by: David S. Miller <davem@davemloft.net>