commit 062c2e43 upstream.

Avoid losing precision.  See bug:
https://bugs.freedesktop.org/show_bug.cgi?id=69675

v2: fix math as per Anssi's comments.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>