hwrng: virtio - remove #ifdef guards for PM functions

Use pm_sleep_ptr for the freeze and restore functions instead of putting
them under #ifdef CONFIG_PM_SLEEP. The resulting code is slightly simpler.

pm_sleep_ptr lets the compiler see the functions but also allows removing
them as unused code if !CONFIG_PM_SLEEP.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>