fm10k: remove comment about rtnl_lock around mbx operations

This comment is no longer true due to a couple of mailbox locking
refactors, and we now don't actually do any rtnl protected operations
directly in the mailbox path. Remove this comment as it is factually
incorrect and confusing.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>