qseecom: Ensure incoming "app_name" does not corrupt the kernel stack

Printing a string with that does not have null terminated character,
would lead to overflow, as the print continues until it finds a null
terminated character.
Avoid this issue by explicitly assigning a string with null termination.

Change-Id: I9528db2ba046c514d829097d08c09540588bb1a2
Signed-off-by: Hariprasad Dhalinarasimha <hnamgund@codeaurora.org>