The Synopsys TSN MAC supports Qbv base times in the past, but only up to a
certain limit. As a result, a taprio qdisc configuration with a small
base time (for example when treating the base time as a simple phase
offset) is not applied by the hardware and silently ignored.

This was observed on an NXP i.MX8MPlus device, but likely affects all
TSN-variants of the MAC.

Fix the issue by making sure the base time is in the future, pushing it by
an integer amount of cycle times if needed. (a similar check is already
done in several other taprio implementations, see for example
drivers/net/ethernet/intel/igc/igc_tsn.c#L116 or
drivers/net/dsa/sja1105/sja1105_ptp.h#L39).

Fixes: b60189e0 ("net: stmmac: Integrate EST with TAPRIO scheduler API")
Signed-off-by: Yannick Vignon <yannick.vignon@nxp.com>
Link: https://lore.kernel.org/r/20210113131557.24651-2-yannick.vignon@oss.nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>