[mips] remove check that is always false (unsinged < 0)

cpuid and local cpu variable are unsigned so checking if value is less than zero
always yields false.

PR:		211088
Submitted by:	David Binderman <dcb314@hotmail.com>
MFC after:	1 week

sys/mips/mips/tlb.c

@@ -348,7 +348,7 @@ DB_SHOW_COMMAND(tlb, ddb_dump_tlb)
 	else
 		cpu = PCPU_GET(cpuid);
 
-	if (cpu < 0 || cpu >= mp_ncpus) {
+	if (cpu >= mp_ncpus) {
 		db_printf("Invalid CPU %u\n", cpu);
 		return;
 	}