diag: Initialize spin lock once per memory device channel

Initialize the spin lock only once per memory device channel
while initializing the channel and channel buffers.

Change-Id: Iabf2db03436102310ef85f04b59e6160d49ef5f5
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>