net: ipa: simplify endpoint transaction completion

When a GSI transaction completes, ipa_endpoint_trans_complete() is
eventually called.  That handles TX and RX completions separately,
but ipa_endpoint_tx_complete() is a no-op.

Instead, have ipa_endpoint_trans_complete() return immediately for a
TX transaction, and incorporate code from ipa_endpoint_rx_complete()
to handle RX transactions.

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