msm: ipa: Fix deleting the TX napi context

TX napi context is a mutable variable and so must be deleted
whenever the pipe is disconnected. Otherwise we will see a
memory leak.

Change-Id: I9309130c786e1092fd1a717359774fca9a37fcd1
Signed-off-by: Chaitanya Pratapa <cpratapa@codeaurora.org>