drm/amdkfd: fix improper return value on error

In function kfd_wait_on_events(), when the call to copy_from_user()
fails, the value of return variable ret is 0. 0 indicates success, which
is inconsistent with the execution status. This patch fixes the bug by
assigning "-EFAULT" to ret when copy_from_user() returns an unexpected
value.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>