net: fec: switch from asm/cacheflush.h to linux/cacheflush.h

To fix the checkpatch warning, use linux/cacheflush.h instead of
asm/cacheflush.h.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Reviewed-by: Wei Fang <wei.fang@nxp.com>
Link: https://patch.msgid.link/20250618-fec-cleanups-v4-3-c16f9a1af124@pengutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>