Input: tsc2005 - do not use 0 in place of NULL

Sparse in unhappy when people use 0 instead of NULL for pointers so
let's rework the way we initialize spi_transfer structure in
tsc2005_cmd() and tsc2005_write().

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