serial: dz: Enable modular build

Enable modular build since the driver now has a proper module_exit()
handler.  There's nothing specific to DZ hardware to prevent driver
unloading and reloading from working.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://patch.msgid.link/alpine.DEB.2.21.2605062331420.46195@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: maciej-w-rozycki

drivers/tty/serial/Kconfig

@@ -334,7 +334,7 @@ config SERIAL_MAX310X
 	  Say Y here if you want to support this ICs.
 
 config SERIAL_DZ
-	bool "DECstation DZ serial driver"
+	tristate "DECstation DZ serial driver"
 	depends on MACH_DECSTATION && 32BIT
 	select SERIAL_CORE
 	default y