8250_mid uses rational_best_approximation() function, so the
driver needs to select CONFIG_RATIONAL option.

This fixes build error when CONFIG_RATIONAL is not enabled:

drivers/built-in.o: In function `mid8250_set_termios':
8250_mid.c:(.text+0x10169a): undefined reference to `rational_best_approximation'
Reported-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NHeikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: NAndy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>