fm10k: rename mbx_tx_oversized statistic to mbx_tx_dropped

Originally this statistic was renamed because the method of dropping was
called "drop_oversized_messages", but this logic has changed much, and
this counter does actually represent messages which we failed to
transmit for a number of reasons. Rename the counter back to tx_dropped
since this is when it will increment, and it is less 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>