net/mlx5: fix assert in doorbell lookup

net/mlx5: fix assert in doorbell lookup

The asserts makes sure that 'i' doesn't exceed the expected value.
This to prevent an out of bound access to dbr_bitmap.

The current location of the assert protects the assignment of
dbr_bitmap, but not the access to it.

Moved the assert to the correct place, to protect both cases.
Also, used an existing define for the assert.

Fixes: 21cae8580fd0 ("net/mlx5: allocate door-bells via DevX")
Cc: stable@dpdk.org
Signed-off-by: Asaf Penso <asafp@mellanox.com>
Reviewed-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>