When an RX transaction completes, we update the trans->len field to
contain the actual number of bytes received.  This is done in a loop
in gsi_evt_ring_rx_update().

Change that function so it checks the data transfer direction
recorded in the transaction, and only updates trans->len for RX
transfers.

Then call it unconditionally.  This means events for TX endpoints
will run through the loop without otherwise doing anything, but
this will change shortly.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>