i2c: designware-baytrail: another fixup for proper Kconfig dependencies

IOSF_MBI is tristate. Baytrail driver isn't.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: David E. Box <david.e.box@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>