msm: ipa: nullify client_notify before disconnecting tethering protocol

After stopping a channel and before disconnecting a tethering
protocol, the EP's client_notify CB should be NULL so the CB is
not called by IPA driver when the protocol is disconnected.

Change-Id: I27ced2f5e498e68294d3dce98ccedd82437c7e68
Signed-off-by: Sivan Reinstein <sivanr@codeaurora.org>