bnxt_en: Free port stats during firmware reset.

Firmware is unable to retain the port counters during any kind of
fatal or non-fatal resets, so we must clear the port counters to
avoid false detection of port counter overflow.

Fixes: fea6b333 ("bnxt_en: Accumulate all counters.")
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Reviewed-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>