8250_mid.c:undefined reference to `rational_best_approximation'

[kbuild test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   8005c49d9aea74d382f474ce11afbbc7d7130bec
commit: d9eda9bab237259b06690652b145d19e0ce37a77 serial: 8250_pci: Intel MID UART support to its own driver
date:   4 weeks ago
config: x86_64-randconfig-s1-11161029 (attached as .config)
reproduce:
        git checkout d9eda9bab237259b06690652b145d19e0ce37a77
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All errors (new ones prefixed by >>):

   drivers/built-in.o: In function `mid8250_set_termios':
>> 8250_mid.c:(.text+0xf089f): undefined reference to `rational_best_approximation'

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