escc: Update RR0 if no character is available.

The RR0_RX_CHARACTER_AVAILABLE bit is cleared after the character is read in receive_byte, but clear it anyway in read_reg when there's no character available just in case a character that was available somehow becomes unavailable.

Author: joevt

devices/serial/escc.cpp

@@ -331,7 +331,9 @@ uint8_t EsccChannel::read_reg(int reg_num)
     switch (reg_num) {
     case RR0:
         if (this->chario->rcv_char_available()) {
-            this->read_regs[RR0] |= RR0_RX_CHARACTER_AVAILABLE;
+            return this->read_regs[RR0] |= RR0_RX_CHARACTER_AVAILABLE;
+        } else {
+            return this->read_regs[RR0] &= ~RR0_RX_CHARACTER_AVAILABLE;
         }
         break;
     case RR8: