PPC: ePAPR: Fix hypercall on LE guest

We get an array of instructions from the hypervisor via device tree that
we write into a buffer that gets executed whenever we want to make an
ePAPR compliant hypercall.

However, the hypervisor passes us these instructions in BE order which
we have to manually convert to LE when we want to run them in LE mode.

With this fixup in place, I can successfully run LE kernels with KVM
PV enabled on PR KVM.

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