net/mlx5: Devcom, only use devcom after NULL check in mlx5_devcom_send_event()

There is a warning reported by kernel test robot:

smatch warnings:
drivers/net/ethernet/mellanox/mlx5/core/lib/devcom.c:264
	mlx5_devcom_send_event() warn: variable dereferenced before
		IS_ERR check devcom (see line 259)

The reason for the warning is that the pointer is used before check, put
the assignment to comp after devcom check to silence the warning.

Fixes: 88d162b4

 ("net/mlx5: Devcom, Infrastructure changes")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/r/202308041028.AkXYDwJ6-lkp@intel.com/
Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>