input: touchscreen: validate bounds of intr_reg_num

Validate the intr_reg_num value returned by touchscreen
to ensure no out of bounds access can occur.

Bug: 35472278
Signed-off-by: Andrew Chant <achant@google.com>

Change-Id: Ib2b5a53034fd1306d29a74ff0f2f698f149fc8c3