hp-wmi: Enable hotkeys on some systems

This is a third attempt to enable these buttons. The new variable being
commit 997daa1b

 (i.e. hp-wmi: detect
"2009 BIOS or later"). Older systems that do not have the 2009 BIOS query
method respond with a dummy value, in this case 4. Using that, we can
target a fairly narrow group of systems. i.e. old enough to not have
HPWMI_FEATURE_QUERY 0xd, but new enough to have HPWMI_BIOS_QUERY 0x9.
This group may be further limited if some systems respond with something
other than 4 to non-existant feature queries.

Signed-off-by: Kyle Evans <kvans32@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>