input: synaptics_dsx: 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
Change-Id: I70f0215d6e9a6bf42efad33e2d9d8e6f745be5a1
Signed-off-by: Andrew Chant <achant@google.com>