x86 platform drivers: hp-wmi Reorder event id processing

Event id 0x4 defines the hotkey event.
No need (or even wrong) to query HPWMI_HOTKEY_QUERY if event id is != 0x4.

Reorder the eventcode conditionals and use switch case instead of if/else.
Use an enum for the event ids cases.

Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
CC: linux-acpi@vger.kernel.org
CC: platform-driver-x86@vger.kernel.org