greybus: loopback: Move latency_ts initialization to transfer routine

Initializing the bridge specific latency variables is only relevant to the
transfer operation, so make it loopback-transfer specific.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>