If driver determines that extended TX port statistics are not supported
or allocation of the data structure fails, make sure to pass 0 TX stats
size to firmware to disable it.  The firmware returned TX stats size should
also be set to 0 for consistency.  This will prevent
bnxt_get_ethtool_stats() from accessing the NULL TX stats pointer in
case there is mismatch between firmware and driver.

Fixes: 36e53349 ("bnxt_en: Add additional extended port statistics.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>