PPC: SPE: Fix high-bits bitmask

The SPE emulation code wants to access the highest 32bits of a 64bit register
and uses the andi TCG instruction for that. Unfortunately it masked with the
wrong mask. Fix the mask to actually cover the upper 32 bits.

This fixes simple multiplication tests with SPE guests for me.

Signed-off-by: Alexander Graf <agraf@suse.de>