msm: ADSPRPC: validate context pointer with magic number

Validate context pointer using magic number instead of searching
through context list. It removes the usage of spin lock in interrupt
handler for avoiding deadlock and reducing latency.

Change-Id: I2492a7984a8d6545618a9cfb7a2d239d03ddd5a2
Acked-by: Viswanatham Paduchuri <vpaduchu@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>