util/spkmodem-recv: add a usage function

replace the err call in getopt

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/spkmodem_recv/spkmodem-recv.c

@@ -71,6 +71,7 @@ static void print_stats(struct decoder_state *st);
 static void reset_char(struct decoder_state *st);
 
 static void err(int errval, const char *msg, ...);
+static void usage(void);
 static const char *progname(void);
 
 int getopt(int, char * const *, const char *);
@@ -110,7 +111,7 @@ main(int argc, char **argv)
 			st.debug = 1;
 			break;
 		default:
-			err(EINVAL, "Invalid argument");
+			usage();
 		}
 	}
 
@@ -279,6 +280,13 @@ err(int errval, const char *msg, ...)
 	exit(EXIT_FAILURE);
 }
 
+static void
+usage(void)
+{
+	fprintf(stderr, "usage: %s [-d]\n", progname());
+	exit(EXIT_FAILURE);
+}
+
 static const char *
 progname(void)
 {