net: sun: sungem: rix a possible null dereference

The function gem_begin_auto_negotiation dereference
the pointer ep before testing if it's null. This
patch add a check on ep before dereferencing it.

Fixes: 92552fdd ("net: sun: sungem: use new api
ethtool_{get|set}_link_ksettings")

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Philippe Reynes <tremyfr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>