Fix hgatp CSR write

The write mask is incorrectly computed by hgatp_csr_t::unlogged_write()
which leads to hgatp.PPN bits not getting updated upon hgatp CSR write.

This patch fixes hgatp CSR writes and gets KVM RISC-V working again
on Spike.

Fixes: 70b7e9ca

 ("mmu: support asid/vmid (#928)")
Signed-off-by: Anup Patel <anup@brainfault.org>