Keep count of number of shortfalls.

To enable this, we keep a count of the current shortfall length. Each
time an SGPIO read/write cannot be completed due to a shortfall, we
increase this length. Each time an SGPIO read/write is completed
successfully, we reset the shortfall length to zero.

When a shortfall occurs and the existing shortfall length is zero, this
indicates a new shortfall, and the shortfall count is incremented.

This change adds one cycle to the normal RX & TX paths, to zero the
shortfall count. To enable this to be done in a single cycle, we keep a
zero handy in a high register.

The extra accounting adds 10 cycles to the TX and RX shortfall paths,
plus an additional 3 cycles to the RX shortfall path since there are
now two branches involved: one to the shortfall handler, and another
back to the main loop.

host/libhackrf/src/hackrf.h

@@ -163,6 +163,8 @@ typedef struct {
 	uint32_t m0_count;
 	/** Number of bytes transferred by the M4. */
 	uint32_t m4_count;
+	/** Number of shortfalls. */
+	uint32_t num_shortfalls;
 } hackrf_m0_state;
 
 struct hackrf_device_list {