drivers: sdhc: Fix C++ compilation

Fix invalid references to the nonexistent `_cplusplus` definition.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>
diff --git a/include/zephyr/drivers/sdhc.h b/include/zephyr/drivers/sdhc.h
index f0368f3..8fb0861 100644
--- a/include/zephyr/drivers/sdhc.h
+++ b/include/zephyr/drivers/sdhc.h
@@ -21,7 +21,7 @@
  * @{
  */
 
-#ifdef _cplusplus
+#ifdef __cplusplus
 extern "C" {
 #endif
 
@@ -429,7 +429,7 @@
  * @}
  */
 
-#ifdef _cplusplus
+#ifdef __cplusplus
 }
 #endif