subsys/net wifi/wifi_shell: Fix use of sys_getopt global status

sys_getopt has now its global status variables prefixed with sys_getopt_
Let's fix the use here. Note this worked as Zephyr's POSIX_C_LIB_EXT
keeps a replica of these variables without the prefix.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>