soc: qcom: subsystem_notif_virt: Only initialize work for virtual systems

A function is assigned to a work struct only when the subsystem
is a virtual subsystem.

Change-Id: Icdb3ddba4509d3727ecb634ccc6108c3b3b2a05d
Signed-off-by: Anant Goel <anantg@codeaurora.org>