Replace M0 state offset field with a byte count.

Instead of this count wrapping at the buffer size, it now increments
continuously. The offset within the buffer is now obtained from the
lower bits of the count.

This makes it possible to keep track of the total number of bytes
transferred by the M0 core.

The count will wrap at 2^32 bytes, which at 20Msps will occur every
107 seconds.

host/hackrf-tools/src/hackrf_debug.c

@@ -379,7 +379,7 @@ int write_register(hackrf_device* device, uint8_t part,
 
 static void print_state(hackrf_m0_state *state) {
 	printf("M0 state:\n");
-	printf("Offset: %u bytes\n", state->offset);
+	printf("M0 count: %u bytes\n", state->m0_count);
 	printf("TX: %u\n", state->tx);
 }