msm: adsprpc: remove use of vfs_stat API in the fastrpc driver

Currently, to check if a particular DSP is supported, vfs_stat on
its device file is done in the kernel. Remove the usage of this
API for GKI compliance.

Change-Id: I5660ddd975354563bf319cefe931d5eac488191e
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>
Signed-off-by: Himateja Reddy <hmreddy@codeaurora.org>