net: sparx5: replace sparx5_start() with sparx5_forwarding_init()

net: sparx5: replace sparx5_start() with sparx5_forwarding_init()

With all subsystem initializations moved out, sparx5_start() only sets
up forwarding (UPSIDs, CPU ports, masks, PGIDs, FCS, watermarks).
Rename it to sparx5_forwarding_init() and make it void since it cannot
fail. This removes sparx5_start() entirely.

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://patch.msgid.link/20260227-sparx5-init-deinit-v2-9-10ba54ccf005@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>