diag: Synchronize command registration table access

Currently, command registration table is being read
in debugfs without any protection which may lead to
access of stale entries. The patch takes care of the
issue by adding proper protection.

CRs-Fixed: 2032672
Change-Id: I6ae058c16873f9ed52ae6516a1a70fd6d2d0da80
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>