media: dvb: mpq: Fix stream-buffer read operation

Data should be read from the buffer according to the size
requested from the user or the actual amount of data
available, whichever is smaller

Change-Id: I81e6a2c538644417caeba9c19060f58810c1fdc3
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>