net: fec: allow disable coalescing

net: fec: allow disable coalescing

In the current implementation, IP coalescing is always enabled and
cannot be disabled.

As setting maximum frames to 0 or 1, or setting delay to zero implies
immediate delivery of single packets/IRQs, disable coalescing in
hardware in these cases.

This also guarantees that coalescing is never enabled with ICFT or ICTT
set to zero, a configuration that could lead to unpredictable behaviour
according to i.MX8MP reference manual.

Signed-off-by: Jonas Rebmann <jre@pengutronix.de>
Reviewed-by: Wei Fang <wei.fang@nxp.com>
Link: https://patch.msgid.link/20250626-fec_deactivate_coalescing-v2-1-0b217f2e80da@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>