remsim_client: Fix custom ATR '-a' option

So far the option only parsed the hexstring into the atr buffer, but
failed to update the length (from the default value of 2). Also set
atr.len so sending custom ATRs works.

Change-Id: I5deb0a432c05ab0bab5081a2812f6718d4a2ac33

src/client/remsim_client_main.c

@@ -117,6 +117,7 @@ static void handle_options(struct client_config *cfg, int argc, char **argv)
 				fprintf(stderr, "ATR malformed\n");
 				exit(2);
 			}
+			cfg->atr.len = rc;
 			break;
 		case 'r':
 			cfg->atr_ignore_rspro = true;