can: mcp251xfd: mcp251xfd_handle_rxif_ring_uinc(): factor out in separate function

This is a preparation patch.

Sending the UINC messages followed by incrementing the tail pointer
will be called in more than one place in upcoming patches, so factor
this out into a separate function.

Also make mcp251xfd_handle_rxif_ring_uinc() safe to be called with a
"len" of 0.

Tested-by: Stefan Althöfer <Stefan.Althoefer@janztec.com>
Tested-by: Thomas Kopp <thomas.kopp@microchip.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>