usb: isp1760: Fix out-of-bounds array access

usb: isp1760: Fix out-of-bounds array access

Running the driver through kasan gives an interesting splat:

  BUG: KASAN: global-out-of-bounds in isp1760_register+0x180/0x70c
  Read of size 20 at addr f1db2e64 by task swapper/0/1
  (...)
  isp1760_register from isp1760_plat_probe+0x1d8/0x220
  (...)

This happens because the loop reading the regmap fields for the
different ISP1760 variants look like this:

  for (i = 0; i < HC_FIELD_MAX; i++) { ... }

Meaning it expects the arrays to be at least HC_FIELD_MAX - 1 long.

However the arrays isp1760_hc_reg_fields[], isp1763_hc_reg_fields[],
isp1763_hc_volatile_ranges[] and isp1763_dc_volatile_ranges[] are
dynamically sized during compilation.

Fix this by putting an empty assignment to the [HC_FIELD_MAX]
and [DC_FIELD_MAX] array member at the end of each array.
This will make the array one member longer than it needs to be,
but avoids the risk of overwriting whatever is inside
[HC_FIELD_MAX - 1] and is simple and intuitive to read. Also
add comments explaining what is going on.

Fixes: 1da9e1c06873 ("usb: isp1760: move to regmap for register access")
Cc: stable@vger.kernel.org
Cc: Rui Miguel Silva <rui.silva@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220516091424.391209-1-linus.walleij@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>