pata_acpi: fix build breakage if !CONFIG_PM

There are configurations where CONFIG_ACPI but !CONFIG_PM.  In this
case, pata_acpi can be selected but won't build.  Fix it.

Reported by Avuton Olrich.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Avuton Olrich <avuton@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>