boot_serial: Fix rc not being returned as a signed value

Fixes an issue whereby rc is a signed variable but is returned as
an unsigned variable in the zcbor functions.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/boot/boot_serial/src/boot_serial.c b/boot/boot_serial/src/boot_serial.c
index 21a9507..4de72ec 100644
--- a/boot/boot_serial/src/boot_serial.c
+++ b/boot/boot_serial/src/boot_serial.c
@@ -552,7 +552,7 @@
     BOOT_LOG_INF("RX: 0x%x", rc);
     zcbor_map_start_encode(cbor_state, 10);
     zcbor_tstr_put_lit_cast(cbor_state, "rc");
-    zcbor_uint32_put(cbor_state, rc);
+    zcbor_int32_put(cbor_state, rc);
     if (rc == 0) {
         zcbor_tstr_put_lit_cast(cbor_state, "off");
         zcbor_uint32_put(cbor_state, curr_off);
@@ -578,7 +578,7 @@
 {
     zcbor_map_start_encode(cbor_state, 10);
     zcbor_tstr_put_lit_cast(cbor_state, "rc");
-    zcbor_uint32_put(cbor_state, rc_code);
+    zcbor_int32_put(cbor_state, rc_code);
     zcbor_map_end_encode(cbor_state, 10);
     boot_serial_output();
 }