ACPI: EC: Make acpi_ec_dsdt_probe() more straightforward

Since acpi_ec_dsdt_probe() returns early if boot_ec is set, it is
always unset when that function calls acpi_config_boot_ec() (passing
ec->handle as the handle argument to it).  Thus it is not really
useful to call acpi_config_boot_ec() at that point.  It is sufficient to
call acpi_ec_setup() directly and (if that is successful) set boot_ec,
so make acpi_ec_dsdt_probe() do that and avoid some pointless checks
in acpi_config_boot_ec().

No intentional functional impact except for a changed message.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>