V4L/DVB (3438): Optical cleanup for dmxdev.c

Used Lindent, manually changed some line breaks. Removed invalid email
addresses, useless casts and useless initialization of return values.
Signed-off-by: NAndreas Oberritter <obi@linuxtv.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab@infradead.org>