[POWERPC] Fix viodasd driver with scatterlist debug

The iSeries viodasd drivers does some very strange things with
scatterlists, one of these causing a BUG_ON to trigger when
scatterlist debugging is enabled due to initializing the
scatterlist with memset instead of sg_init_table().

This fixes it by using sg_init_table().  The rest of the stuff
it does to that poor list is still pretty awful but it will work.

I may look into fixing things in a nicer way some other time.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>