uart: uart_getenv: check for NULL class last, not first

This allows one to specify dt:XXXX when the default class isn't compiled
into the kernel. It's not an error to not have a class until we're done
parsing the spec, so defer checking until then.

Sponsored by:		Netflix
Reviewed by:		adrian, andrew, markj
Differential Revision:	https://reviews.freebsd.org/D47078

sys/dev/uart/uart_subr.c

@@ -200,13 +200,6 @@ uart_getenv(int devtype, struct uart_devinfo *di, struct uart_class *class)
 	bus_addr_t addr = ~0U;
 	int error;
 
-	/*
-	 * All uart_class references are weak. Make sure the default
-	 * device class has been compiled-in.
-	 */
-	if (class == NULL)
-		return (ENXIO);
-
 	/*
 	 * Check the environment variables "hw.uart.console" and
 	 * "hw.uart.dbgport". These variables, when present, specify
@@ -300,6 +293,13 @@ uart_getenv(int devtype, struct uart_devinfo *di, struct uart_class *class)
 		goto inval;
 	freeenv(cp);
 
+	/*
+	 * The default uart_class reference is weak. Make sure the default
+	 * device class has been compiled-in or we've set one with dt=.
+	 */
+	if (class == NULL)
+		return (ENXIO);
+
 	/*
 	 * Accept only the well-known baudrates. Any invalid baudrate
 	 * is silently replaced with a 0-valued baudrate. The 0 baudrate