media: stkwebcam: Use more common logging styles

Convert STK_<LEVEL> to pr_<level> to use the typical kernel logging.
Add a define for pr_fmt.  No change in logging output.

Miscellanea:

o Remove now unused PREFIX and STK_<LEVEL> macros
o Realign arguments
o Use pr_<level>_ratelimited
o Add a few missing newlines to formats

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>