drivers/staging/rtl8192u/r8192U_wx.c: fix warnings issued by sparse

This minor patch motivated by eudyptula challenge fixes the following warnings issued
by `sparse' in drivers/staging/rtl8192u/r8192U_wx.c:
 .../r8192U_wx.c:27:5: warning: symbol 'rtl8180_rates' was not declared. Should it be static?
 .../r8192U_wx.c:961:22: warning: symbol 'r8192_get_wireless_stats' was not declared. Should it be static?
 .../r8192U_wx.c:990:24: warning: symbol 'r8192_wx_handlers_def' was not declared. Should it be static?

Signed-off-by: Ovidiu Toader <ovi@phas.ubc.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>