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: Ieba53b123d6553c5381db9f0b6d57b8f343e6155
Acked-by: Vishnu Karthik D <vikarthi@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>