Check __must_check warnings for class_device_register and class_device_create_file

video_device_create_file was declared as a void, but instead should
return the int value of class_device_create_file.

Move the check from bttv-driver.c into v4l2-dev.h, because all other
callers of video_device_create_file must also be checked.

Replace the call to class_device_create_file in videodev.c with
video_device_create_file, as defined in v4l2-dev.h, so that the
return value of class_device_create_file will be checked.

Check the return value of class_device_register in videodev.c and
pvrusb2-sysfs.c
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>