V4L/DVB (5052): Pvrusb2: Remove stream claiming hack from /dev/radio

Trying to temporarily check that the stream is not claimed during open
of the radio device is at best a race condition.  What's to stop
another app from claiming the stream anyway the instant after the
check is done?  The implementation for this was dicey anyway.  So it's
removed.  The only "price" for this is that if /dev/radioX is opened
while streaming video, then the video stream is just going to switch
to radio mode anyway.  If a user does this, he gets what he expects...

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>