serial: msm_serial: Make config tristate

This driver works as a module, so mark it as a tristate config
instead of a bool.

Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Reviewed-by: Andy Gross <agross@codeaurora.org>
Signed-off-by: Andy Gross <agross@codeaurora.org>

drivers/tty/serial/Kconfig

@@ -1044,7 +1044,7 @@ config SERIAL_SGI_IOC3
 	  say Y or M.  Otherwise, say N.
 
 config SERIAL_MSM
-	bool "MSM on-chip serial port support"
+	tristate "MSM on-chip serial port support"
 	depends on ARCH_QCOM
 	select SERIAL_CORE