usb: tcpm: fix inverted poll condition in tcpm_pd_transmit()

The read_poll_timeout() macro breaks out of its loop when the condition
evaluates to true. The current code uses "!tx_complete" as the
condition, which means it exits immediately when tx_complete is false
(i.e., transmission has NOT completed yet), rather than waiting for
completion.

Fix the condition to "tx_complete" so that the poll loop waits until
the TCPC signals transmission success/failure/discard before
proceeding.

Without this fix, tcpm_pd_transmit() returns before the TCPC has
finished transmitting, causing the PD state machine to proceed with
stale tx_status values.

Fixes: 1db4c0ac77e3 ("usb: tcpm: add core framework")
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>