ata: let compiler optimize out ata_eh_set_lpm() on non-SATA hosts

Add !IS_ENABLED(CONFIG_SATA_HOST) to ata_eh_set_lpm() to allow
compiler to optimize out the function for non-SATA configs (for
PATA hosts "ap && !ap->ops->set_lpm" condition is always true so
it's sufficient for the function to return zero).

Code size savings on m68k arch using (modified) atari_defconfig:

   text    data     bss     dec     hex filename
before:
  17353      18       0   17371    43db drivers/ata/libata-eh.o
after:
  16607      18       0   16625    40f1 drivers/ata/libata-eh.o

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/ata/libata-eh.c

@@ -3443,7 +3443,8 @@ static int ata_eh_set_lpm(struct ata_link *link, enum ata_lpm_policy policy,
 	int rc;
 
 	/* if the link or host doesn't do LPM, noop */
-	if ((link->flags & ATA_LFLAG_NO_LPM) || (ap && !ap->ops->set_lpm))
+	if (!IS_ENABLED(CONFIG_SATA_HOST) ||
+	    (link->flags & ATA_LFLAG_NO_LPM) || (ap && !ap->ops->set_lpm))
 		return 0;
 
 	/*