drivers: dma_mcux_lpc: Status fixes

Some miscellaneous fixes to LPC DMA driver regarding status tracking:

- If a DMA channel has not been configured for any transfer,
  there will be a bug caused by the virtual channel being -1
  and then trying to index -1 into the driver data structs.
  Add -EACCES return code to indicate this situation.

- Return -EINVAL from get_status if channel number is invalid

- Update the busy flag in the LPC DMA callback function.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>