net/mlx5: Check if lag is supported before creating one

This patch fixes a memleak issue by preventing to create a lag and
add PFs if lag is not supported.

comm “python3”, pid 349349, jiffies 4296985507 (age 1446.976s)
hex dump (first 32 bytes):
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  …………….
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  …………….
 backtrace:
  [<000000005b216ae7>] mlx5_lag_add+0x1d5/0×3f0 [mlx5_core]
  [<000000000445aa55>] mlx5e_nic_enable+0x66/0×1b0 [mlx5_core]
  [<00000000c56734c3>] mlx5e_attach_netdev+0x16e/0×200 [mlx5_core]
  [<0000000030439d1f>] mlx5e_attach+0x5c/0×90 [mlx5_core]
  [<0000000018fd8615>] mlx5e_add+0x1a4/0×410 [mlx5_core]
  [<0000000068bc504b>] mlx5_add_device+0x72/0×120 [mlx5_core]
  [<000000009fce51f9>] mlx5_register_device+0x77/0xb0 [mlx5_core]
  [<00000000d0d81ff3>] mlx5_load_one+0xc58/0×1eb0 [mlx5_core]
  [<0000000045077adc>] init_one+0x3ea/0×920 [mlx5_core]
  [<0000000043287674>] pci_device_probe+0xcd/0×150
  [<00000000dafd3279>] really_probe+0x1c9/0×4b0
  [<00000000f06bdd84>] driver_probe_device+0x5d/0×140
  [<00000000e3d508b6>] device_driver_attach+0x4f/0×60
  [<0000000084fba0f0>] bind_store+0xbf/0×120
  [<00000000bf6622b3>] kernfs_fop_write+0x114/0×1b0

Fixes: 9b412cc3

 ("net/mlx5e: Add LAG warning if bond slave is not lag master")
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>