net: stmmac: allow a tc-taprio base-time of zero

Commit fe28c53e ("net: stmmac: fix taprio configuration when
base_time is in the past") allowed some base time values in the past,
but apparently not all, the base-time value of 0 (Jan 1st 1970) is still
explicitly denied by the driver.

Remove the bogus check.

Fixes: b60189e0 ("net: stmmac: Integrate EST with TAPRIO scheduler API")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>