pch_gbe: fix transmit watchdog timeout

An extended ping test with 6 vlans resulted in a driver oops with a
netdev transmit timeout.
Fix WATCHDOG_TIMEOUT to be more like e1000e at 5 * HZ, to avoid
unnecessary transmit timeouts.

Signed-off-by: Andy Cress <andy.cress@us.kontron.com>
Signed-off-by: David S. Miller <davem@davemloft.net>