diag: Update hdlc disable protection to avoid deadlock

Currently there is a possibility of deadlock while processing
apps data. The patch updates the scope of hdlc_disable_mutex
to avoid the deadlock situation.

Change-Id: I070e1a546a016fdd854ad413c03c0bc6f192c516
Signed-off-by: Hardik Arya <harya@codeaurora.org>