ata: ahci: Clarify mobile_lpm_policy description

Despite its name, the mobile_lpm_policy module parameter defines the
default LPM policy to use for an AHCI adapter for all chipsets,
including desktop and server chipsets. Clarify this point in the
parameter description.

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>