Input: tsc2005 - use spi_get/set_drvdata()

Instead of peeking into underlying device and using dev_get/set_drvdata(),
let's use SPI layer's implementation to access driver-private data
(which may be different from driver-core private data).

Tested-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>