[sw, dif_uart] Fix use of wrong enum in return code

Two functions were returning enum values from the wrong enum type. The
numerical value was the same as for the correct enum, so this was only a
latent bug. This was noticed when compiling with Clang, which warns about
the use of mismatching enums.

Signed-off-by: Luís Marques <luismarques@lowrisc.org>
diff --git a/sw/device/lib/dif/dif_uart.c b/sw/device/lib/dif/dif_uart.c
index ce66a92..951c1fe 100644
--- a/sw/device/lib/dif/dif_uart.c
+++ b/sw/device/lib/dif/dif_uart.c
@@ -170,7 +170,7 @@
                                        const dif_uart_config_t *config,
                                        dif_uart_t *uart) {
   if (uart == NULL || config == NULL) {
-    return kDifUartBadArg;
+    return kDifUartConfigBadArg;
   }
 
   uart->base_addr = base_addr;
@@ -181,7 +181,7 @@
 dif_uart_config_result_t dif_uart_configure(const dif_uart_t *uart,
                                             const dif_uart_config_t *config) {
   if ((uart == NULL) || (config == NULL)) {
-    return kDifUartBadArg;
+    return kDifUartConfigBadArg;
   }
 
   return uart_configure(uart, config);