media: vivid: use vfree() instead of kfree() for dev->bitmap_cap

syzkaller reported crashes on kfree() called from
vivid_vid_cap_s_selection(). This looks like a simple typo, as
dev->bitmap_cap is allocated with vzalloc() throughout the file.

Fixes: ef834f78

 ("[media] vivid: add the video capture and output
parts")

Signed-off-by: Alexander Potapenko <glider@google.com>
Reported-by: Syzbot <syzbot+6c0effb5877f6b0344e2@syzkaller.appspotmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>