cnic: Support NIC Partition mode

Add a common function cnic_read_bnx2x_iscsi_mac() to read the iSCSI
MAC address at any specified shared memory location.  In NIC Partition
mode, we need to get the MAC address from the MF_CFG area of shared
memory.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>