staging: rtl8188eu: remove redundant definition of ETH_ALEN

ETH_ALEN is defined in linux/if_ether.h which is included by
osdep_service.h, so remove the redundant definition from ieee80211.h.

osdep_service.h:33:#include <linux/etherdevice.h>
etherdevice.h:25:#include <linux/if_ether.h>

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>