drivers: dma: siwx91x: Fix burst length processing

1. Corrected the burst length processing to be handled in bytes
   for the siwx917 DMA drivers.
2. Removed overlay and configuration files associated with the
   chan_blen_transfer test application. The chan_blen_transfer
   test application attempted to use 8 and 16 byte bursts, which
   are not supported by the siwx91x UDMA.

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