net: xilinx: axienet: Fix IRQ coalescing packet count overflow

If coalesce_count is greater than 255 it will not fit in the register and
will overflow. This can be reproduced by running

    # ethtool -C ethX rx-frames 256

which will result in a timeout of 0us instead. Fix this by checking for
invalid values and reporting an error.

Fixes: 8a3b7a25 ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")
Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Reviewed-by: Shannon Nelson <shannon.nelson@amd.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Link: https://patch.msgid.link/20250113163001.2335235-1-sean.anderson@linux.dev


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