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: Ibb3d8c4fb637ddc0e63677ec2dff14a4cf8c0c73
Signed-off-by: Srinivasarao P <spathi@codeaurora.org>