ata: ahci: Rename `AHCI_HFLAG_IS_MOBILE`

`AHCI_HFLAG_IS_MOBILE` designates that a chipset should be using the
default link power management policy from a kernel configuration item.

As desktop chipsets may also be interested in this default policy
configuration, rename the flag to `AHCI_HFLAG_USE_LPM_POLICY` to more
accurately reflect that a chipset doesn't have to be mobile to adopt it.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>