IB/mlx4: mlx4_ib_fmr_alloc() should call mlx4_fmr_enable()

Currently mlx4_ib_fmr_alloc() calls mlx4_mr_enable() instead of
mlx4_fmr_enable().  The two functions are equivalent at the moment, but 
this is not really correct (and the change is needed to fix a bug).

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>