irqchip/gic-v3-its: Fix u64 to __le64 warnings

The its_cmd_block struct can either have u64 or __le64
data in it, so make a anonymous union to remove the
sparse warnings when converting to/from these.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20191017112955.15853-1-ben.dooks@codethink.co.uk