hw/timer/renesas_tmr: Add default-case asserts in read_tcnt()

In commit 81b3ddaf8772ec we fixed a use of uninitialized data
in read_tcnt(). However this change wasn't enough to placate
Coverity, which is not smart enough to see that if we read a
2 bit field and then handle cases 0, 1, 2 and 3 then there cannot
be a flow of execution through the switch default. Add explicit
default cases which assert that they can't be reached, which
should help silence Coverity.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210319162458.13760-1-peter.maydell@linaro.org

hw/timer/renesas_tmr.c

@@ -146,6 +146,8 @@ static uint16_t read_tcnt(RTMRState *tmr, unsigned size, int ch)
         case CSS_CASCADING:
             tcnt[1] = tmr->tcnt[1];
             break;
+        default:
+            g_assert_not_reached();
         }
         switch (FIELD_EX8(tmr->tccr[0], TCCR, CSS)) {
         case CSS_INTERNAL:
@@ -159,6 +161,8 @@ static uint16_t read_tcnt(RTMRState *tmr, unsigned size, int ch)
         case CSS_EXTERNAL: /* QEMU doesn't implement this */
             tcnt[0] = tmr->tcnt[0];
             break;
+        default:
+            g_assert_not_reached();
         }
     } else {
         tcnt[0] = tmr->tcnt[0];