net: ipa: introduce ipa_cmd_tag_process()

Create a new function ipa_cmd_tag_process() that simply allocates a
transaction, adds a tag process command to it to clear the hardware
pipeline, and commits the transaction.

Call it in from ipa_endpoint_suspend(), after suspending the modem
endpoints but before suspending the AP command TX and AP LAN RX
endpoints (which are used by the tag sequence).

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>