[media] em28xx-i2c: cleanup I2C debug messages

The I2C output messages is too polluted. Clean it a little
bit, by:
	- use the proper core support for memory dumps;
	- hide most stuff under the i2c_debug umbrella;
	- add the missing KERN_CONT where needed;
	- use 2 levels or verbosity. Only the second one
	  will show the I2C transfer data.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>