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>