board: microchip: icicle: correct type for node offset

Node offsets returned by libfdt can contain negative error numbers, so
the variable type should be "int". As things stand, if the ethernet
nodes are not found in the early init callback, the if (node < 0) tests
pass and the code errors out while trying to set the local-mac-address
for a non-existent node.

Fixes: 64413e1b ("riscv: Add Microchip MPFS Icicle Kit support")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>