vfio/platform: Fix spelling mistake "registe" -> "register"

There is a spelling mistake in a comment, fix it.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20210326083528.1329-5-thunder.leizhen@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>