usbc: check if message is pending before trying to receive it

If TCPC supports checking for pending messages, it should be done
instead of forcefully trying to read the message and getting no-data
result. This helps with powereficiency for chips that go to sleep
when there is no message pending. Trying to read the message can
wake up the chip providing higher power usage.

Signed-off-by: Michał Barnaś <mb@semihalf.com>