boards: arm: nrf52840_pca10090: swap error for assert in board.c
boards: arm: nrf52840_pca10090: swap error for assert in board.c

Use an assert to catch the situation where we can't find a port
to use for the reset line pin, since it shouldn't really happen.

The case where reset_pin_configure() is executed but no reset
line is selected can't happen due to static asserts in `integrity.c`
that enforce a reset line to be selected when the reset pin
functionality is enabled.

Signed-off-by: Emanuele Di Santo <58b3ebaa556fff9ee88aab1044a35f0af0dd7061@nordicsemi.no>

nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr