scmi: refactor the code to hide a channel from devices

The commit 85dc5828 ("firmware: scmi: prepare uclass to pass channel
reference") added an explicit parameter, channel, but it seems to make
the code complex.

Hiding this parameter will allow for adding a generic (protocol-agnostic)
helper function, i.e. for PROTOCOL_VERSION, in a later patch.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>