gpio: bd70528: Use correct unit for debounce times

The debounce time passed to gpiod_set_debounce() is specified in
microseconds, so make sure to use the correct unit when computing the
register values, which denote delays in milliseconds.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Cc: <stable@vger.kernel.org>
Fixes: 18bc64b3 ("gpio: Initial support for ROHM bd70528 GPIO block")
[Bartosz: fixed a typo in commit message]
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>