i3c: mipi-i3c-hci: Extract ring initialization from hci_dma_init()

i3c: mipi-i3c-hci: Extract ring initialization from hci_dma_init()

Split the ring setup logic out of hci_dma_init() into a new helper
hci_dma_init_rings().  This refactoring prepares for Runtime PM support
by allowing DMA rings to be reinitialized independently after resume.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260113072702.16268-11-adrian.hunter@intel.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>