[sw, uart] Fix ConfigTest NullArgs test

This test was testing wrong API (dif_uart_init instead of
dif_uart_configure).

Signed-off-by: Silvestrs Timofejevs <silvestrst@lowrisc.org>
diff --git a/sw/device/tests/dif/dif_uart_test.cc b/sw/device/tests/dif/dif_uart_test.cc
index ffb2eb7..df7b92f 100644
--- a/sw/device/tests/dif/dif_uart_test.cc
+++ b/sw/device/tests/dif/dif_uart_test.cc
@@ -134,9 +134,9 @@
 class ConfigTest : public UartTest {};
 
 TEST_F(ConfigTest, NullArgs) {
-  EXPECT_FALSE(dif_uart_init(base_addr_, &dif_uart_config_, nullptr));
-  EXPECT_FALSE(dif_uart_init(base_addr_, nullptr, &dif_uart_));
-  EXPECT_FALSE(dif_uart_init(base_addr_, nullptr, nullptr));
+  EXPECT_FALSE(dif_uart_configure(nullptr, &dif_uart_config_));
+  EXPECT_FALSE(dif_uart_configure(&dif_uart_, nullptr));
+  EXPECT_FALSE(dif_uart_configure(nullptr, nullptr));
 }
 
 TEST_F(ConfigTest, Default) {