staging: wlan-ng: avoid new typedef: p80211item_pstr255_t

This patch fixes the following checkpatch.pl warning in p80211types.h:
WARNING: do not add new typedefs

It applies for typedef p80211item_pstr255_t

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