bna: Remove get_regs Ethtool Support

Change details:
 - This patch contains removal of get_regs support in bnad_ethtool.c. Thus
   BNA will have minimal register definitions necessary for MBOX and
   interrupt operations

Signed-off-by: Rasesh Mody <rmody@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>