qeth: make local functions static in qeth_l3 module

This patch makes 4 local functions static and removes
the prototypes from the header file.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>