net/mlx5: Check device memory pointer before usage

Add a NULL check before accessing device memory to prevent a crash if
dev->dm allocation in mlx5_init_once() fails.

Fixes: c9b9dcb4 ("net/mlx5: Move device memory management to mlx5_core")
Signed-off-by: Stav Aviram <saviram@nvidia.com>
Link: https://patch.msgid.link/c88711327f4d74d5cebc730dc629607e989ca187.1751370035.git.leon@kernel.org


Signed-off-by: Leon Romanovsky <leon@kernel.org>