media: dvb: increase SDMX shared buffer size

A shared buffer is used to encode the command and response when
sending a command to TZ via qseecom interface.
To allow increasing the number of linear buffers the shared buffer
size is increased to accommodate the command data.
In addition, add checks to detect overflow of the shared buffer.

Change-Id: Ie006d20f3841aa7e8ea2e30bd6a5b211099936af
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>