[media] em28xx: simplify argument passing to em28xx_init_dev()

The 'struct em28xx *' pointer was passed by reference to the
em28xx_init_dev() function, for no reason. Instead, just pass it by
value, which is much more logical and simple.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>