media: uvcvideo: Include streaming interface number in debugfs dir name

uvcvideo creates a debugfs directory based on the device bus number and
device number. If a device contains more than one uvc function, the
creation of the second and following debugfs directories will fail and
print an info message like this:
  "uvcvideo: Unable to create debugfs 3-2 directory."

This patch includes the uvc streaming interface number in the debugfs
directory name, to make sure it is unique. The directory name format is
changed from "<busnum>-<devnum>" to "<busnum>-<devnum>-<intfnum>"

Signed-off-by: Torleiv Sundre <torleiv@huddly.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>