lib: Use correct type for return value

The error check 'plic_off < 0' does nothing, since plic_off is stored as
a u32. Fix this by changing it to an int, which matches the return type of
fdt_node_offset_by_compatible().

Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>