Input: tsc2005 - separate SPI and core functions

This patch separates the SPI functionality from core functionality
that overlaps with the tsc2004.

Prepares kernel for new tsc2004 driver without much redundant code.

Signed-off-by: Michael Welling <mwelling@ieee.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>