diag: Initialize control channel workqueues correctly

Diag driver initializes the control channel workqueues after
registering with SMD. In some cases, the driver can be notified
of channel open before the intialization is complete. This path
initializes the control channel workqueues before registering
with SMD.

Change-Id: If84a22fd1d7113a55b8801e1087a7c9cd986472f
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>