xen/arm: guest_walk: Avoid theoritical unitialized value in get_top_bit

xen/arm: guest_walk: Avoid theoritical unitialized value in get_top_bit

Clang 8.0 throws an error in the get_top_bit function:

guest_walk.c:328:15: error: variable 'topbit' is used uninitialized
whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
    else if ( is_64bit_domain(d) )
              ^~~~~~~~~~~~~~~~~~

This is happening because clang thinks that is_32bit_domain(d) is not
the exact inverse of is_64bit_domain(d). So it expects a else case to
handle the case where the latter call is false.

In other part of the code, dealing with difference between 32-bit and
64-bit domain, we usually use if ( is_XXbit_domain ) ... else ...

So use the same pattern here.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>