net: ipa: introduce gsi_channel_initialized()

Create a simple helper function that indicates whether a channel has
been initialized.  This abstacts/hides the details of how this is
determined.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 6170b6da)