msm: ipa: Fix setting disconnect in progress flag

When USB is disconnected USB driver deregisters callback
with IPA driver. If there are exception packets coming to USB
driver at the same time there is a possiblity of race condition
where callback is de-registered and IPA driver tries to access it.
Make changes to restrict the callback access when disconnect is
in progress.

Change-Id: I85d2876847bfd0b23f19b2b81e1aab68729b2e31
Acked-by: Chaitanya Pratapa <cpratapa@qti.qualcomm.com>
Signed-off-by: Bojun Pan <bojunp@codeaurora.org>