ACPI: PM: s2idle: Rearrange lps0_device_attach()

To allow a subsequent change to be simpler, rearrange the code in
lps0_device_attach() to reduce the indentation level and (while
at it) make it avoid calling lpi_device_get_constraints() when
lps0_device_handle is not going to be set.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>