mhi: dev: uci: fix incorrect return code during open failure

UCI open was returning success when channel was in disable
state. This change return the correct error code.

CRs-Fixed: 2356399
Change-Id: I0f7919324d560ba28a91b17ff2be0b644481d75b
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>