Don't ask twice about not including staging drivers

The "Exclude staging drivers" question is there so that we don't build
staging drivers for allyesconfig or allnoconfig settings, but it's very
irritating when you've already said "no" to staging drivers earlier.

There is absolutely no point in declining twice - once you've declined
the staging drivers, you're done.

So make the second question depend on the first question having been
answered in the affirmative.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>