fm10k: Unlock mailbox on VLAN addition failures

After grabbing the mailbox lock and detecting an error, the lock must be
released before the error code can be returned.

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>