modem: wncm14a2a: Update driver to use gpio_dt_spec

Simplify the implementation by using gpio_dt_spec.

As part of this use logical values of 0/1 for gpio_pin_set_dt
and handle any inversions as GPIO flags that might be required
at a board level (in dts config).

Signed-off-by: Kumar Gala <galak@kernel.org>