sh_eth: align usage of sh_eth_modify() with rest of driver

To be consistent with the rest of the driver when setting bits using
sh_eth_modify() the same bit should also be cleared. This have no
functional change and should have been done from the start.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Suggested-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>