msm: npu: update debugfs functions

This change is to remove register write function from debugfs
since it's not used. And it also fixes some potential buffer
overflow issues.

Change-Id: I96bb47c6a58d5c8b9c57eed8de6000d2c2c3e377
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>