net: ethernet: ti: am65-cpsw: fix CONFIG_PM #ifdef

The #ifdef check is incorrect and leads to a warning:

drivers/net/ethernet/ti/am65-cpsw-nuss.c:1679:13: error: 'am65_cpsw_nuss_remove_rx_chns' defined but not used [-Werror=unused-function]
 1679 | static void am65_cpsw_nuss_remove_rx_chns(void *data)

It's better to remove the #ifdef here and use the modern
SYSTEM_SLEEP_PM_OPS() macro instead.

Fixes: 24bc19b0

 ("net: ethernet: ti: am65-cpsw: Add suspend/resume support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://lore.kernel.org/r/20221215163918.611609-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>