cmd: adc: Report return value on error

Reporting the return value should always be done on error conditions,
this way the developer can start debugging issues with more knowledge
in-hand.

Signed-off-by: Samuel Dionne-Riel <samuel@dionne-riel.com>
diff --git a/cmd/adc.c b/cmd/adc.c
index 16f914a..75739bc 100644
--- a/cmd/adc.c
+++ b/cmd/adc.c
@@ -81,8 +81,8 @@
 	ret = adc_channel_single_shot(argv[1], simple_strtol(argv[2], NULL, 0),
 				      &data);
 	if (ret) {
-		printf("Error getting single shot for device %s channel %s\n",
-		       argv[1], argv[2]);
+		printf("Error getting single shot for device %s channel %s (ret=%d)\n",
+		       argv[1], argv[2], ret);
 		return CMD_RET_FAILURE;
 	}