bnxt_en: Fix HWTSTAMP_FILTER_ALL packet timestamp logic

bnxt_en: Fix HWTSTAMP_FILTER_ALL packet timestamp logic

When the chip is configured to timestamp all receive packets, the
timestamp in the RX completion is only valid if the metadata
present flag is not set for packets received on the wire.  In
addition, internal loopback packets will never have a valid timestamp
and the timestamp field will always be zero.  We must exclude
any 0 value in the timestamp field because there is no way to
determine if it is a loopback packet or not.

Add a new function bnxt_rx_ts_valid() to check for all timestamp
valid conditions.

Fixes: 66ed81dcedc6 ("bnxt_en: Enable packet timestamping for all RX packets")
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/20231208001658.14230-5-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>