iommu/iommu-debug: fix buffer overflows in debugfs read functions

The kernel buffer 'ubuf' can overflow while copying data to user
space in debugfs read functions. Fix it by limiting the length of
data to be copied to userspace.

Change-Id: Ic6d532f013dfab2dcb10650a8db435531a1509bf
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>