usb: pd: Don't print pd_send_msg error if PD not connected

commit 87b3d7aa ("usb: pd: Add string definitions for message
types") added an error message to the log whenever any PD transmit
fails. But this should be suppressed at least for Source Capabilities
when connecting to a non-PD sink. Hence, only print an error message
if pd_connected is true.

Change-Id: Iac4768249fd272c84a398c7f12186127cc9dca0a
Signed-off-by: Jack Pham <jackp@codeaurora.org>