net: ehea: avoid null pointer dereference

ehea_get_port may return NULL. Do not dereference NULL value.

Fixes: 8c4877a4

 ("ehea: Use the standard logging functions")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>