drivers: pinctrl_nrf: Fix disconnecting of pins

This is a follow-up to commit 223cc3c6.

When a peripheral pin is disconnected, the pinctrl driver should skip
applying of GPIO configuration, as there is no pin number available in
such case, but due to an incorrect check, it actually did not skip it
and used an incorrect pin number for that. In nrfx prior to 3.0.0, this
caused an assertion failure, but because of a fallback routine, things
could still work in most cases (when assertions were disabled) as that
GPIO configuration was just applied to P0.31. Hence the bug was not
discovered until now. In the recent nrfx, this causes a null pointer
dereference, so always a crash.
This commit corrects the mentioned check and also uses the term "psel"
instead of "pin" where it is possible that the value is not a correct
pin number, in the hope of preventing a similar problem in the future.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>