BF can be disabled in some cases, the capability field, bf_reg_size is set
to zero in this case. Don't map the BF area in this case, it would cause
failures.  In addition, leaving the BF area unmapped
also alerts the ETH driver to not use BF.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>