Add mobus code guard

Author: engigeer

expanders_init.h

@@ -67,9 +67,8 @@ extern void pca9654e_init(void);
 
 #if PICOHAL_IO_ENABLE
 
-#ifndef MODBUS_ENABLE
-#error "Modbus must be enabled to use the Picohal IO expander!"
-#undef PICOHAL_IO_ENABLE
+#if !MODBUS_ENABLE
+#error "Modbus must be enabled to use the PicoHAL IO expander!"
 #endif
 
 extern void picohal_io_init (void);