bnxt_en: Pass correct extended TX port statistics size to firmware.

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>