platform/chrome: Kconfig: Remove CONFIG_ prefix from MFD_CROS_EC section

Remove the CONFIG_ prefix from the select statement for MFD_CROS_EC.

Fixes: 2fa2b980

 ("mfd / platform: cros_ec: Rename config to a better name")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>