msm: adsprpc: disable bind/unbind of fastrpc driver from userspace

Set attribute to disable binding and unbinding of fastrpc driver
from userspace.

Change-Id: I3f81850fd07e77251599907aa3b4bc93c1ee9224
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>