net: dsa: Fix build warning for !PM_SLEEP

The dsa_switch_suspend() and dsa_switch_resume() functions are only used
when PM_SLEEP is enabled, so they need #ifdef CONFIG_PM_SLEEP protection
to avoid a compiler warning.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>