iommu: iommu-debug: don't pass null character to copy_to_user

Doing a cat on pte,test_virt_addr from adb shell prints a null
character as well which is at the end of the string this is not
required so, don't pass this null character to copy_to_user.

Change-Id: I8d9120f64d1df84a704379eb00bd239fc7059e9e
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>