staging: r8188eu: remove dev_power_down function prototype

The dev_power_down function is not present in the r8188eu driver. Remove
its prototype.
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20230111195640.306748-10-martin@kaiser.cxSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/r8188eu/include/rtw_io.h

@@ -206,6 +206,4 @@ void _rtw_write_mem(struct adapter *adapter, u32 addr, u32 cnt, u8 *pmem);
 u32 rtw_write_port(struct adapter *adapter, u32 addr, u32 cnt, u8 *pmem);
 void rtw_write_port_cancel(struct adapter *adapter);
 
-void dev_power_down(struct adapter *Adapter, u8 bpwrup);
-
 #endif	/* _RTL8711_IO_H_ */