drivers: dma: siwx917: peripheral req & burst length function changes

- siwx917_is_peripheral_request -> siwx917_transfer_direction
- siwx917_burst_length -> siwx917_is_burst_length_valid
- siwx917_burst_length return type changed to bool
- Added enum for siwx917_transfer_direction return values

Signed-off-by: Sai Santhosh Malae <Santhosh.Malae@silabs.com>