[nrf fromlist] debug: coresight: cs_trace_defmt: Fix uninitialized variable warning

In certain configurations there could be a warning due to potential
use of uninitialized variable. Add initialization.

Upstream PR #: 107183

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

Author: nordic-krch

subsys/debug/coresight/cs_trace_defmt.c

@@ -25,7 +25,7 @@ int cs_trace_defmt_process(const uint8_t *data, size_t len)
 
 	uint8_t aux = data[15];
 	uint8_t d_id;
-	uint8_t cb_id;
+	uint8_t cb_id = 0;
 	bool do_cb = false;
 
 	for (int i = 0; i < 8; i++) {