drivers: mfd: fix dependencies for MFD_RTSX_PCI

We need to include MFD_CORE if this option is enabled, otherwise we get
build errors.

Cc: Wei WANG <wei_wang@realsil.com.cn>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/mfd/Kconfig

@@ -66,6 +66,7 @@ config MFD_SM501_GPIO
 config MFD_RTSX_PCI
 	tristate "Support for Realtek PCI-E card reader"
 	depends on PCI
+	select MFD_CORE
 	help
 	  This supports for Realtek PCI-Express card reader including rts5209,
 	  rts5229, rtl8411, etc. Realtek card reader supports access to many