ata: Kconfig: Update SATA_MOBILE_LPM_POLICY default to med_power_with_dipm

Historically this was set to "keep_firmware_settings" to prevent problems
with power management on very old drives. However it's been observed that
almost all modern Linux distributions either set the policy to
"med_power_with_dipm" in the kernel configuration or update it to this via
userspace policy changes. Update the policy default in the kernel to
"med_power_with_dipm" to match that behavior as well.

Changing the default was previously not a good idea, because LPM disables
detection of hot plug removals, however, since commit ae1f3db0 ("ata:
ahci: do not enable LPM on external ports"), a port marked as external
will always be initialized to "keep_firmware_settings", regardless of the
SATA_MOBILE_LPM_POLICY Kconfig value. Therefore, changing the default is
now considered safe (external ports included).

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
[cassel: rebased and reworded commit message]
Signed-off-by: Niklas Cassel <cassel@kernel.org>