fm10k/base: avoid Tx drop increment during mailbox negotiation

fm10k/base: avoid Tx drop increment during mailbox negotiation

After shutting down the mailbox by force, we then go about resetting max
size to 0, and clearing all messages in the FIFO. However, we should
just reset the head pointer so that the FIFO will become 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: Wang Xiao W <xiao.w.wang@intel.com>