drm/sun4i: Fix an ulong overflow in the dotclock driver

drm/sun4i: Fix an ulong overflow in the dotclock driver

commit e84cb605e02f1b3d0aee8d7157419cd8aaa06038 upstream.

The calculated ideal rate can easily overflow an unsigned long, thus
making the best div selection buggy as soon as no ideal match is found
before the overflow occurs.

Fixes: 4731a72df273 ("drm/sun4i: request exact rates to our parents")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181018100250.12565-1-boris.brezillon@bootlin.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>