Keep track of longest shortfall.

This adds six cycles to the TX and RX shortfall paths.

host/hackrf-tools/src/hackrf_debug.c

@@ -389,6 +389,7 @@ static void print_state(hackrf_m0_state *state) {
 	printf("M0 count: %u bytes\n", state->m0_count);
 	printf("M4 count: %u bytes\n", state->m4_count);
 	printf("Number of shortfalls: %u\n", state->num_shortfalls);
+	printf("Longest shortfall: %u bytes\n", state->longest_shortfall);
 }
 
 static void usage() {