net: xilinx: axienet: Report an error for bad coalesce settings

Instead of silently ignoring invalid/unsupported settings, report an
error. Additionally, relax the check for non-zero usecs to apply only
when it will be used (i.e. when frames != 1).

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Reviewed by: Shannon Nelson <shannon.nelson@amd.com>
Link: https://patch.msgid.link/20250116232954.2696930-3-sean.anderson@linux.dev


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