powerpc/kvm: Force cast endianness of KVM shared regs

Sparse reports endianness mismatches in the KVM shared regs getter and
setter helpers.

This code has dynamic endianness behind a safe interface, so a force is
warranted here to tell sparse this is OK.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Link: https://msgid.link/20231011053711.93427-8-bgray@linux.ibm.com