msm: ADSPRPC: Free contexts for current channel

When multiple channels are opened from same process, free
contexts associated only with the current channel when the
device is released.

Change-Id: Iaa1f06ee00f3b9420ef5b00995bdf8186cb83283
Acked-by: Sathish Ambley <sambley@qti.qualcomm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>