igbvf: add tx_timeout_count to ethtool statistics

Add `tx_timeout_count` to ethtool statistics to provide visibility into
transmit timeout events, bringing igbvf in line with other Intel
ethernet drivers.

Currently `tx_timeout_count` is incremented in igbvf_watchdog_task() and
igbvf_tx_timeout() but is not exposed to userspace nor used elsewhere in
the driver.

Before:
  # ethtool -S ens5 | grep tx
       tx_packets: 43
       tx_bytes: 4408
       tx_restart_queue: 0

After:
  # ethtool -S ens5 | grep tx
       tx_packets: 41
       tx_bytes: 4241
       tx_restart_queue: 0
       tx_timeout_count: 0

Tested-by: Kohei Enju <enjuk@amazon.com>
Signed-off-by: Kohei Enju <enjuk@amazon.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>