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>