IB/mlx4: deprecate "failed to alloc bf reg" message from err to debug

IB/mlx4: deprecate "failed to alloc bf reg" message from err to debug

This message is not an error, and qp creation continues
normally -- only without use of blueflame.

For VFs attached to VMs, KVM disables write combining, so
performance is better without BF in this case.  The host driver
therefore intentionally disables BF use for guests. (see upstream
kernel commit b91cb3ebcd).

The message notifying that BF is not available is therefore
deprecated from err to debug.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
(Ported from Mellanox OFED 2.4)

Signed-off-by: Mukesh Kacker <mukesh.kacker@oracle.com>