input: touchscreen: remove global variable from ITE tech touch driver

Remove the use of global variable from ITE tech touch driver,
and use the internal driver structure passed as function argument.
This change makes the driver complied to kernel coding standards
and similar to other touch driver.

CRs-Fixed: 982219
Change-Id: Ib609d6e76ea70cd3b49c4ea6f09c75bf52521aa8
Signed-off-by: Shantanu Jain <shjain@codeaurora.org>