qeth: Clear mac_bits field when switching between Layer 2 and Layer 3

This patch fixes a problem that occurs when switching from layer 3 to layer 2
mode. Resetting this mac_bits makes sure that we retrieve our mac address from
the card, otherwise the interface simply would'nt work.

Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>