net/mlx5: Fix dump_command mailbox length printed

Dump command mailbox length printed was correct only if data_only flag
was set. For the case that data_only flag was clear the offset to stop
printing at was wrong and so the buffer printed was too short.
Changed the print loop to stop according to number of buffers in
mailbox.

Fixes: e126ba97 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>