e1000e: e1000e_enable_tx_pkt_filtering() returns wrong value

e1000e_enable_tx_pkt_filtering() will return a non-zero value if the
driver fails to enable the manageability interface on the host for
any reason; instead it should retun zero to indicate filtering has been
disabled.  Also provide a single exit point for the function.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>