bnxt: count Tx drops

Drivers should count packets they are dropping.

Fixes: c0c050c5

 ("bnxt_en: New Broadcom ethernet driver.")
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>