msm: ADSPRPC: Substitute vfs check with flags

To check if DSP is supported or not, we make
fs call for subsystem device node. This node
is not accessible to untrusted applications.
Use subsystem status flag instead to avoid
permission issues and return proper error
in case subsystem is not up.

Change-Id: Ia19e31b899600e5d765c0a3582bdf9132c9b67bf
Acked-by: Ekansh Gupta <ekangupt@qti.qualcomm.com>
Signed-off-by: Vamsi krishna Gattupalli <vgattupa@codeaurora.org>