pch_can: Fix warnings

Currently, in case CONFIG_PM is disabled, compiler outputs warnings.
Move six functions which are used only CONFIG_PM is enabled,
into "#ifdef CONFIG_PM" area.

Signed-off-by: Tomoya MORINAGA <tomoya-linux@dsn.okisemi.com>
Signed-off-by: David S. Miller <davem@davemloft.net>