ata: libata-eh: Add ata_eh_set_lpm() WARN_ON_ONCE

link->lpm_policy is initialized to ATA_LPM_UNKNOWN in ata_eh_reset().

ata_eh_set_lpm() is then only called if
link->lpm_policy != ap->target_lpm_policy (after reset)

and then only if link->lpm_policy > ATA_LPM_MAX_POWER (before
revalidation).

This means that ata_eh_set_lpm() is currently never called with
policy == ATA_LPM_UNKNOWN.

Add a WARN_ON_ONCE so that it is more obvious from reading the code that
this function is never called with policy == ATA_LPM_UNKNOWN.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>