usbc: check if sampling CC lines were successful

When the get_cc function returns error, the CC lines should be
considered as open. It may happen either due to some chip malfunction
or chip going into sleep mode and needing time to reinitialize.
TCPC should only go to sleep if there is no partner attaches, so
in case of this error, the lines should be considered as disconnected.

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