cxgb4: remove redundant assignment to vlan_cmd.dropnovlan_fm

A recent commit updated vlan_cmd.dropnovlan_fm but failed to remove
the older assignment.  Fix this by removing the former redundant
assignment.

Detected by CoverityScan, CID#1473290 ("Unused value")

Fixes: a89cdd8e ("cxgb4: impose mandatory VLAN usage when non-zero TAG ID")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>