staging:r8188eu: use DIV_ROUND_UP() in rtw_signal_stat_timer_hdl()

DIV_ROUND_UP macro is shorter and look better than if-else construction.
DIV_ROUND_UP used in rtw_signal_stat_timer_hdl().

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>