msm: ipa: fix tx completion crash during disconnect

If a client does not wait till all pending TX packets are
completed before disconnecting the pipe, a crash is seen
during IPA driver TX completion of those pending packets.
This commit fixes that crash.

Change-Id: I2791dd055e80943f668b5c1453dc584aa65021cb
Signed-off-by: Ravi Gummadidala <rgummadi@codeaurora.org>