media: dvb: Add buffer flushing API

Automatically flushing the output buffer when buffer overflows is
not always the desired behavior as this discards pending data while
user may actually want to read the data collected so far.
Add a new module parameter to allow turning off the automatic
buffer flushing as well as a new ioctl DMX_FLUSH_BUFFER to allow
user to flush the output buffer manually.

Change-Id: I191bfcdd1e3194cd47884f0eabbbaf36b0a44f7e
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>