drivers: media: broadcast: Fix security vulnerability issue

Information leak issue is reported in mpq_sdmx_log_level_write
function. Added check to validate count is not zero and initialize
the string.

Change-Id: Ieb2ed88c2d7d778c56be2ec3b9875270a9c74dce
Signed-off-by: Udaya Bhaskara Reddy Mallavarapu <udaym@codeaurora.org>