greybus: arche-platform: Rework platform/wd-line state transition logic

This patch simplifies and improves the readability of the internal state
transition logic of arche platform driver state transition logic by:

1. Making a dedicated function to state-transition the platform code.
2. Condense the decision to swtich states down to a single switch
   statement instead of four separate if/else clauses.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Suggested-by: Alex Elder <elder@linaro.org>
Reviewed-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>