acpi_spmc(4): Small probe improvements/fixes

Remove the test on presence of an ACPI handle, this is implied by
ACPI_ID_PROBE() succeeding.

Set 'sc->dev' early, so that acpi_spmc_check_dsm_set() using
device_printf() will print the driver name.

Add a missing newline after printing that more DSM functions are
implemented then expected.

Reviewed by: obiwac
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56483