media: dvb: Fix memory leak in DMX_ADD_PID

Adding a new PID to a recording filter while it is already running
might fail, in which case the dmxdev_feed object that was allocated is
not freed.

Change-Id: I7d322570fd3e17a82ab60a8d5f59077018094ce3
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>