[media] easycap: use usb_kill_urb wrapper functions

1. kill_video_usb can be used in all places where video urbs are killed
and reduce code repetition

2. remove unnecessary check for easycap == NULL in the function
as it is always checked by the calling function

3. rename the function to easycap_video_kill_urb to reduce
possibility of name conflict

4. implement also easycap_audio_kill_urb

5. simplify freeing urbs

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>