powerpc: Use sizeof(*foo) rather than sizeof(struct foo)

It's slightly less error prone to use sizeof(*foo) rather than
specifying the type.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
[mpe: Consolidate into one patch, rewrite change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>