input: touchscreen: fix issue raised by static analyzer in maxim_sti driver

Fix issue raised by static analyzer in maxim_sti touch driver.
Initialize the "id" variable properly in driver function, which is
used to specify the touch event id.

CRs-fixed: 995978
Change-Id: Ib8e730980d07cb28dc1ac6124476620d79bd374f
Signed-off-by: Shantanu Jain <shjain@codeaurora.org>