usb: pd: Further differentiate errors from select_pdo_store

Currently -EINVAL is returned both when a select_pdo string is
improperly formulated as well as when an RDO request is
successfully sent but outright rejected or otherwise failed
to enter a new contract. To allow userspace to differentiate
these two failures, instead return -ECONNREFUSED in the latter
case.

Change-Id: I74378b469745d0ef414f8cae228f79741d4846a5
Signed-off-by: Jack Pham <jackp@codeaurora.org>