V4L/DVB (9668): em28xx: fix a race condition with hald

Newer versions of hald tries to open it to call QUERYCAP.

Due to the lack of a proper locking, it is possible to open the device
before it finishes initialization.

This patch adds a lock to avoid this risk, and to protect the list of
em28xx devices.

While here, remove the uneeded BKL lock.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>