Input: st1232 - do not allocate fingers data separately

The finger structure size is quite small and allocating it together with
the main driver structure will not increase likelyhood of allocation
failing, but reduces number of objects needing to be tracked by the
allocator and devm.

Tested-by: Matthias Fend <Matthias.Fend@wolfvision.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>