x86: use a fallback dev for i386

We can use a fallback dev for cases of a NULL device being passed (mostly ISA)
This comes from x86_64 implementation.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>