[kernel] Fix clock::spinwait

spinwait wasn't scaling the target to microseconds

src/kernel/clock.cpp

@@ -16,7 +16,7 @@ clock::clock(uint64_t rate, clock::source source_func, void *data) :
 void
 clock::spinwait(uint64_t us) const
 {
-	uint64_t when = m_source(m_data) + us;
+	uint64_t when = value() + us;
 	while (value() < when);
 }