input: synaptics_dsx: add update bounds checks.

Firmware updates contain offsets that are parsed
by the kernel driver.  Ensure all offsets are within
the bounds of the firmware update.

This is a secondary patch for the dsx version of the
driver.  Both drivers are loaded at boot.

Bug: 31525965
Bug: 31968442
Change-Id: Icbe28aed0917ddfd089285ab25dbc5826a15be2f
Signed-off-by: Andrew Chant <achant@google.com>