pc_serial: Fix TTYSETRTS

Same fix as hrev42151 for usb_serial:
When determining the bit to be set for RTS/DTR there was a missing comparison
to op which would've caused a wrong line state to be set.

src/add-ons/kernel/drivers/ports/pc_serial/SerialDevice.cpp

@@ -314,7 +314,7 @@ SerialDevice::Service(struct tty *tty, uint32 op, void *buffer, size_t length)
 		case TTYSETRTS:
 		{
 			bool set = *(bool *)buffer;
-			uint8 bit = TTYSETDTR ? MCR_DTR : MCR_RTS;
+			uint8 bit = op == TTYSETDTR ? MCR_DTR : MCR_RTS;
 			if (set)
 				OrReg8(MCR, bit);
 			else