m68k: use fallback for random_get_entropy() instead of zero

In the event that random_get_entropy() can't access a cycle counter or
similar, falling back to returning 0 is really not the best we can do.
Instead, at least calling random_get_entropy_fallback() would be
preferable, because that always needs to return _something_, even
falling back to jiffies eventually. It's not as though
random_get_entropy_fallback() is super high precision or guaranteed to
be entropic, but basically anything that's not zero all the time is
better than returning zero all the time.

Cc: Thomas Gleixner <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Acked-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Jason A. Donenfeld <[e-mail]>

arch/m68k/include/asm/timex.h

···
 {
 	if (mach_random_get_entropy)
 		return mach_random_get_entropy();
-	return 0;
+	return random_get_entropy_fallback();
 }
 #define random_get_entropy	random_get_entropy