[kernel] Fix clock period vs frequency error

Calling `spinwait()` was hanging due to improper computation of the
clock rate because justin did a dumb at math. Also the period can be
greater than 1ns, so the clock's units were updated to microseconds.

src/kernel/clock.h

@@ -12,13 +12,13 @@ public:
 	using source = uint64_t (*)(void*);
 
 	/// Constructor.
-	/// \arg rate   Number of source ticks per ns
+	/// \arg rate   Number of source ticks per us
 	/// \arg source Function for the clock source
 	/// \arg data   Data to pass to the source function
 	clock(uint64_t rate, source source_func, void *data);
 
 	/// Get the current value of the clock.
-	/// \returns Current value of the source, in ns
+	/// \returns Current value of the source, in us
 	/// TODO: optimize divison by finding a multiply and
 	/// shift value instead
 	inline uint64_t value() const { return m_source(m_data) / m_rate; }
@@ -28,15 +28,15 @@ public:
 	inline void update() { m_current = value(); }
 
 	/// Wait in a tight loop
-	/// \arg interval  Time to wait, in ns
-	void spinwait(uint64_t ns) const;
+	/// \arg interval  Time to wait, in us
+	void spinwait(uint64_t us) const;
 
 	/// Get the master clock
 	static clock & get() { return *s_instance; }
 
 private:
-	uint64_t m_current; ///< current ns count
-	uint64_t m_rate; ///< source ticks per ns
+	uint64_t m_current; ///< current us count
+	uint64_t m_rate; ///< source ticks per us
 	void *m_data;
 	source m_source;