fixes signed-unsigned comparison warning

Author: epsilonrt

src/Modbus.cpp

@@ -313,7 +313,7 @@ void Modbus::writeMultipleRegisters (byte * frame, word startreg, word numoutput
   }
 
   //Check Address (startreg...startreg + numregs)
-  for (int k = 0; k < numoutputs; k++) {
+  for (word k = 0; k < numoutputs; k++) {
     if (!this->searchRegister (startreg + 40001 + k)) {
       this->exceptionResponse (MB_FC_WRITE_REGS, MB_EX_ILLEGAL_ADDRESS);
       return;
@@ -544,7 +544,7 @@ void Modbus::writeMultipleCoils (byte * frame, word startreg, word numoutputs, b
   }
 
   //Check Address (startreg...startreg + numregs)
-  for (int k = 0; k < numoutputs; k++) {
+  for (word k = 0; k < numoutputs; k++) {
     if (!this->searchRegister (startreg + 1 + k)) {
       this->exceptionResponse (MB_FC_WRITE_COILS, MB_EX_ILLEGAL_ADDRESS);
       return;