powerpc: Remove `have_of' global variable

The `have_of' variable is a relic from the arch/ppc time, it isn't
useful nowadays.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>