diag: Response for 0x7C command

If modem proc has registered for the polling command and is up,
in that case Apps diag should not respond for the 0x7C command.
The 0x7C response should come from modem.

Change-Id: If1f79d19647988d4d909d1a079c46699c7b3ddab
CRs-fixed: 605172
Signed-off-by: Katish Paran <kparan@codeaurora.org>