fm10k: clarify action when updating the VLAN table

Clarify the comment for when entering promiscuous mode that we update
the VLAN table. Add a comment distinguishing the case where we're
exiting promiscuous mode and need to clear the entire VLAN table.

Signed-off-by: Ngai-Mint Kwan <ngai-mint.kwan@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@gmail.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>