net: sparx5: fix resource_size.cocci warnings

Use resource_size function on resource object
instead of explicit computation.

Clean up coccicheck warning:
./drivers/net/ethernet/microchip/sparx5/sparx5_main.c:237:19-22: ERROR:
Missing resource_size with iores [ idx ]

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>