shell: Fix a typo when adding surface to pending list

shell: Fix a typo when adding surface to pending list

As both names are similar this was probably a typo, and instead of using
the iterator for the list we should use the item itself. This was pretty
hard to track as initially everything was set-up alright.

We were still getting empty lists of pending remote surfaces, when I
observed that we were using the iterator rather than the new item we
just create in that function.

Bug-AGL: SPEC-4445
Reported-by: Scott Murray <scott.murray@konsulko.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I77c4c6721eb4a4e3e8e66faa4fa0584b136d322e