nvethernet: assign macsec private data to NULL

Issue:
L4T platforms on boot comes up with MTU as 1500.
However on changing the MTU to jumbo, the final
MTU which gets set accounts the MACSEC ICV tag
len and reduces the MTU by 34 bytes even though
the platform does not support MACSEC.

Fix:
Assign macsec allocated private data to NULL and
use the NULL check in ndo mtu change ioctl call
to reject MTU reduction if macsec is not supported.

Bug 4137597

Change-Id: Ica3003b54762dd5d240c9b0c5c012049b4e503b2
Signed-off-by: Sushil Kumar Singh <sushilkumars@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nv-oot/+/2928569
Reviewed-by: Revanth Kumar Uppala <ruppala@nvidia.com>
Reviewed-by: Bibek Basu <bbasu@nvidia.com>
Tested-by: Revanth Kumar Uppala <ruppala@nvidia.com>
GVS: Gerrit_Virtual_Submit <buildbot_gerritrpt@nvidia.com>

drivers/net/ethernet/nvidia/nvethernet/macsec.c

@@ -1437,6 +1437,7 @@ car_err:
 	macsec_release_platform_res(macsec_pdata);
 init_err:
 	devm_kfree(dev, pdata->macsec_pdata);
+	pdata->macsec_pdata = NULL;
 exit:
 	PRINT_EXIT();
 	return ret;