V4L/DVB (6538): em28xx: fix locking to allow accesses from 2 different threads at...

V4L/DVB (6538): em28xx: fix locking to allow accesses from 2 different threads at the same time

The attached patch modifies the em28xx driver so that there can be ioctls from
multiple different threads.

This is necessary for capture apps like MPlayer that use different threads for
capturing and channel tuning.

Now the locking is only done for the ioctls that change properties of the
device or access the i2c bus.

It also removes some locks that look unnecessary:

In em28xx_init_dev:
  the videodevice is not registered yet so nothing can access the hardware
 meanwhile, the device struct is not assigned to the interface yet so no race
 with disconnect is possible

In em28xx_release_resources:
  it gets only called when dev->lock is already held

Signed-off-by: Sascha Sommer <saschasommer@freenet.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>