serial: max310x: Allow driver to be compiled as module

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -289,7 +289,7 @@ config SERIAL_MAX3100
 	  MAX3100 chip support
 
 config SERIAL_MAX310X
-	bool "MAX310X support"
+	tristate "MAX310X support"
 	depends on SPI_MASTER
 	select SERIAL_CORE
 	select REGMAP_SPI if SPI_MASTER