msm: ADSPRPC: Null check for file session context

When any system call is made, before servicing it make sure that the
session context is not null and channel info is valid.

Change-Id: I07ff325083610c758f889cbc582dab9c2aa97d46
Acked-by: Vishnu Karthik D <vikarthi@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>