media: dvb: Use SDMX load flag for demux plugin unloading

Unloading the demux plugin checks whether SDMX module is loaded or not
by trying to open a new SDMX session. When SDMX is known not to be
available there is not need to do that. Instead, just check the SDMX
loaded flag which was updated earlier.

Change-Id: I7cb9f7bcfb156c410d2c153fa463b8ef168bed7a
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>