media: atomisp: Remove loading mutex

Now that the registering of the /dev/* video / subdev nodes has been
moved to the end of atomisp_pci_probe() the workaround with the loading
mutex to delay opens until init is done is no longer necessary.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>