dma: dma_nxp_edma: refactor state transitioning

The channel state transitions are currently performed at the
beginning of each of the functions that triggers them
(e.g: edma_start(), edma_stop(), etc...).  The main issue with
this approach is the fact if there's any failures after the state
transition then the channel will be in the target state without
performing the required steps for it.

For instance, during edma_config(), if any of the functions after
the state transition (the channel_change_state() call) fails
(e.g: get_transfer_type()) fails then the state of the channel
will be CONFIGURED even if not all the required steps were performed
(e.g: setting the MUX, configuring the transfer, etc...).

To fix this, split the state transition into two steps:

	1) Check if the transition is possible.
	2) Do the transition.

First step should be done before any configurations to make sure
that we should be performing them in the first place, while the
second step should be performed after all configurations, thus
guaranteeing that all the required steps for the target state were
performed before transitioning to it.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>