bus: mhi: ep: Demote unsupported channel error log to debug

Not all MHI endpoints will support all available channels. Most of them
support only a selected number of channels based on the implementations.
In those cases, it is not needed to print error messages in the endpoint.
So let's demote the error log to debug.

Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>