fm10k: reset head instead of calling update_max_size

When we forcefully shutdown the mailbox, we then go about resetting max
size to 0, and clearing all messages in the FIFO. Instead, we should
just reset the head pointer so that the FIFO becomes empty, rather than
changing the max size to 0. This helps prevent increment in tx_dropped
counter during mailbox negotiation, which is confusing to viewers of
Linux ethtool statistics output.

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