net: ethoc: Demote packet dropped error message to debug

Spamming the console with: net eth1: packet dropped can happen
fairly frequently if the adapter is busy transmitting, demote the
message to a debug print.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>