plic: Fix cpuid handling

plic: Fix cpuid handling

Make `cpu' signed so that the possible return value -1 from
plic_get_cpuid() gets handled correctly in the (cpu < 0) condition.
This prevents plic_attach() from updating sc_contexts[] out of bounds.

When plic_get_cpuid() returns -1, ignore the entry and continue
processing. The error is not fatal. It is normal that secondary CPUs
are not found when running a non-MULTIPROCESSOR kernel on
a multiprocessor machine.

OK kettenis@