goldfish_bus: switch GOLDFISH_BUS config back to bool

AOSP commit a61e4d27 "Enable adb with android-pipe
in IA image" made GOLDFISH_BUS modular (i.e switched it
to "tristate" in Kconfig from "bool"), which is not
buildable. Instead of fixing the build and supporting
it in modular form, which AOSP doesn't agree with anyway,
lets restore it to the original state ("bool") in the Kconfig.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>