Staging: rtl8187se: r8180_wx.c: make 'rtl8180_rates' static

Fixes the following sparse warning:
27:5: warning: symbol 'rtl8180_rates' was not declared. Should it be static?

Signed-off-by: Anmol Sarma <me@anmolsarma.in>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>