staging: wlan-ng: do not return -ENOSYS

Fixed coding style issue by replacing ENOSYS with EINVAL.

This patch fix the following checkpatch.pl warnings in p80211netdev.c:
WARNING: ENOSYS means 'invalid syscall nr' and nothing else

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>