HSI: nokia-modem: Replace of_gpio_count() by gpiod_count()

As a preparation to unexport of_gpio_named_count(), convert the
driver to use gpiod_count() instead.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>