[media] em28xx: convert it from pr_foo() to dev_foo()

Instead of using pr_foo(), use dev_foo(), with provides a
better output. As this device is a multi-interface one,
we'll set the device name to show the chipset and the driver
used.

While here, get rid of printk continuation messages.

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