net: macb: Validate the value of base_time properly

In macb_taprio_setup_replace(), the value of start_time is being
compared against zero which would never be true since start_time
is an unsigned value. Due to this there is a chance that an
incorrect config base time value can be used for computation.

Fix by checking the value of conf->base_time directly.
This issue was reported by static coverity analyzer.

Fixes: 89934dbf ("net: macb: Add TAPRIO traffic scheduling support")
Signed-off-by: Chandra Mohan Sundar <chandramohan.explore@gmail.com>
Reviewed-by: Vineeth Karumanchi <vineeth.karumanchi@amd.com>
Link: https://patch.msgid.link/20250901162923.627765-1-chandramohan.explore@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>