msm: ADSPRPC: Check for valid context during release

Check whether context has been properly assigned during device
open and bail out without calling dsp release if fastrpc
context is NULL, as dsp release fail is causing excessive
logging in kernel logs

Change-Id: I2f77d6b22ed3507a7d5b920fa9ae8af51c0140a1
Acked-by: Himateja Reddy <hmreddy@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>