libata: suppress two warnings

drivers/ata/libata-core.c:768: warning: 'ata_lpm_enable' defined but not used
drivers/ata/libata-core.c:784: warning: 'ata_lpm_disable' defined but not used

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jeff Garzik <jeff@garzik.org>