ata: libata-eh: Move and rename ata_eh_set_lpm()

Move the definition of the function ata_eh_set_lpm() to avoid its
unnecessary forward declaration and rename the function to
ata_eh_link_set_lpm() to clarify that it acts on a link.
No functional changes.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Niklas Cassel <cassel@kernel.org>