pch_gbe: ethtool cannot change parameters when link is down

When attempting to change e.g. the advertising mask when the link is down
ecmd->speed is -1 causing mii_ethtool_sset() to bail out.

This bug bit when connecting to a gigabit switch through a 4-pin (industrial)
cable, since link negotiation would not complete (both endpoints claimed to
be gigabit-capable, but this is not possible with only 4 pins).
Any attempt to fix this by setting autonegation to not offer 1000Mbps
failed as the setting would not be accepted while the link was still down...

Set ecmd->speed to SPEED_1000 to satisfy mii_ethtool_sset()
(the actual value of ecmd->speed doesn't matter as long as it is valid,
since a re-negotation is forced afterwards).

Signed-off-by: Anders Larsen <al@alarsen.net>
Signed-off-by: David S. Miller <davem@davemloft.net>