msm: ipa3: unify xDCI connect & disconnect APIs

Instead of having separate APIs for allocating GSI channels and starting
the channels, have only one API ipa_usb_xdci_connect() that allocates and
starts the GSI channels.
Instead of having separate APIs for deallocating GSI channels and stopping
the channels, have only one API ipa_usb_xdci_disconnect() that deallocates
and stops the GSI channels.
This change is due to race conditions raised by the separation of the APIs
in USB connect/disconnect scenarios.

Change-Id: I9dd243aeb8621325b4e45767d29f9167cdd21007
Signed-off-by: Sivan Reinstein <sivanr@codeaurora.org>