staging: rtl8192su: fix compile error from wireless-testing commit

In wireless-testing, commit 7044cc56 added struct ieee80211_hdr_3addr
to include/linux/ieee80211.h. This definition collides with one that is
in the rtl8192su driver in staging.

The conflict is resolved by changing rtl8192su
to use the definition from include/linux/ieee80211.h.

Signed-off-by: John W. Linville <linville@tuxdriver.com>