touchscreen: Fix sign bug

platform_get_irq_byname() can return negative results, it is not seen to
unsigned ts_irq. Make it signed.

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Acked-By: Luotao Fu <l.fu@pengutronix.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>