staging: rtl8187se: Deleted rtl8225z2_rf_set_mode function

After the next warning messages by sparse tools:

drivers/staging/rtl8187se/r8180_rtl8225z2.c:609:6: warning: symbol 'rtl8225z2_rf_set_mode' was not declared. Should it be static?
drivers/staging/rtl8187se/r8180_rtl8225z2.c:609:13: warning: ‘rtl8225z2_rf_set_mode’ defined but not used [-Wunused-function]
 static void rtl8225z2_rf_set_mode(struct net_device *dev)

And the next search:
~/opw/dev/staging/drivers/staging/rtl8187se
 (staging-next) $ git grep rtl8225z2_rf_set_mode
r8180_rtl8225z2.c:void rtl8225z2_rf_set_mode(struct net_device *dev)

I decided to delete rtl8225z2_rf_set_mode function.

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>