net: wifi: Fix compile error when -Werror -Wextra are set

The warning which became error looks like this

error: type qualifiers ignored on function return type
                                [-Werror=ignored-qualifiers]
  219 | const char * const wifi_ps_txt(enum wifi_ps ps_name);

It is pointless to add a const qualifier to a return value.
So remove the const pointer to avoid this warning.

Fixes #64197

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>