Input: keyboard: Fix ChromeOS EC keyboard help message.

The cros-ec I2C and SPI transport drivers have been moved from MFD
subsystem to platform/chrome, at the same time, the config symbol
has been renamed and lost the MFD_ prefix So, update the help message
accordingly.
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Benson Leung <bleung@chromium.org>

drivers/input/keyboard/Kconfig

@@ -721,7 +721,7 @@ config KEYBOARD_CROS_EC
 	help
 	  Say Y here to enable the matrix keyboard used by ChromeOS devices
 	  and implemented on the ChromeOS EC. You must enable one bus option
-	  (MFD_CROS_EC_I2C or MFD_CROS_EC_SPI) to use this.
+	  (CROS_EC_I2C or CROS_EC_SPI) to use this.
 
 	  To compile this driver as a module, choose M here: the
 	  module will be called cros_ec_keyb.