Input: tsc2005 - simplify drvdata acquisition

Using dev_*_drvdata() instead of spi_*_drvdata() reduces lines of code and
prepares the driver for possible tsc2004 support, which is i2c based.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>