UBUNTU: SAUCE: msm: adsprpc: Don't create debugfs global file if already exists

The adsprpc driver creates the debugfs global file for each instance
of msm_fastrpc_compute_cb but it fails to create after the first one.
This change will check the existence of the file before it attempts
to create the new one.

Change-Id: I90fa44071a9baeca43213561943ed6830977f82c
Signed-off-by: Anuj Garg <anujgarg@codeaurora.org>
(cherry picked from commit 77a641b1099a4cdeed3b47d71ec7553b4affb8ac)
Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>