Add a shortfall length limit.

This limit allows implementing a timeout: if a TX underrun or RX overrun
continues for the specified number of bytes, the M0 will revert to idle.

A setting of zero disables the limit.

This change adds 5 cycles to the TX & RX shortfall paths, to check if a
limit is set and to check the shortfall length against the limit.

host/hackrf-tools/src/hackrf_debug.c

@@ -390,6 +390,7 @@ static void print_state(hackrf_m0_state *state) {
 	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);
+	printf("Shortfall limit: %u bytes\n", state->shortfall_limit);
 }
 
 static void usage() {