diag: Add missing "get log mask" command

DIAG service responds to "get log mask" command sent by tools.
This was typically responded to by modem. On modem less devices,
this command still needs response. Adding support on Linux processor
for this command.

CRs-Fixed: 388740
Change-Id: Id94645ae21117e9952b852855401eadd060279ee
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>