serial: 8250_mid: Add missing dependency

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: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Heikki Krogerus

drivers/tty/serial/8250/Kconfig

@@ -373,6 +373,7 @@ config SERIAL_8250_MID
 	depends on SERIAL_8250 && PCI
 	select HSU_DMA if SERIAL_8250_DMA
 	select HSU_DMA_PCI if X86_INTEL_MID
+	select RATIONAL
 	help
 	  Selecting this option will enable handling of the extra features
 	  present on the UART found on Intel Medfield SOC and various other