V4L/DVB (8894): pvrusb2: Remove BKL

The earlier change from Hans Verkuil that pushed the BKL from
video_open() down into the drivers should be unneeded for the pvrusb2
driver.  This driver's implementation for open already protects its
internal structures through other means, thus the BKL is not required.
This change reverses Hans' previous change, for the pvrusb2 driver.

It probably would have been a good idea for Hans to previously have
asked for my ack before committing his change.

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