RDMA/mlx5: Don't assume that medium blueFlame register exists

User can leave system without medium BlueFlames registers,
however the code assumed that at least one such register exists.

This patch fixes that assumption.

Fixes: c1be5232 ("IB/mlx5: Fix micro UAR allocator")
Reported-by: Rohit Zambre <rzambre@uci.edu>
Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>