net: sparx5: split sparx5_fdma_{start(),stop()}

net: sparx5: split sparx5_fdma_{start(),stop()}

The two functions: sparx5_fdma_{start(),stop()} are responsible for a
number of things, namely: allocation and initialization of FDMA buffers,
activation FDMA channels in hardware and activation of the NAPI
instance.

This patch splits the buffer allocation and initialization into init and
deinit functions, and the channel and NAPI activation into start and
stop functions. This serves two purposes: 1) the start() and stop()
functions can be reused for lan969x and 2) prepares for future MTU
change support, where we must be able to stop and start the FDMA
channels and NAPI instance, without free'ing and reallocating the FDMA
buffers.

Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://patch.msgid.link/20250113-sparx5-lan969x-switch-driver-5-v2-2-c468f02fd623@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>