atm: simplify procfs code

Use remove_proc_subtree to remove the whole subtree on cleanup, and
unwind the registration loop into individual calls.  Switch to use
proc_create_seq where applicable.

Also don't bother handling proc_create* failures - the driver works
perfectly fine without the proc files, and the cleanup will handle
missing files gracefully.

Signed-off-by: Christoph Hellwig <hch@lst.de>