net/mlx5: SD, Handle possible devcom ERR_PTR

Check if devcom holds an error pointer and return immediately.

This fixes Smatch static checker warning:
drivers/net/ethernet/mellanox/mlx5/core/lib/sd.c:221 sd_register()
error: 'devcom' dereferencing possible ERR_PTR()

Enhance mlx5_devcom_register_component() so it stops returning NULL,
making it easier for its callers.

Fixes: d3d05766

 ("net/mlx5: SD, Implement devcom communication and primary election")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/all/f09666c8-e604-41f6-958b-4cc55c73faf9@gmail.com/T/
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Link: https://lore.kernel.org/r/20240411115444.374475-3-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>