ravb: avoid unused function warnings

When CONFIG_PM_SLEEP is disabled, we get a couple of harmless warnings:

drivers/net/ethernet/renesas/ravb_main.c:2117:12: error: 'ravb_resume' defined but not used [-Werror=unused-function]
drivers/net/ethernet/renesas/ravb_main.c:2104:12: error: 'ravb_suspend' defined but not used [-Werror=unused-function]

The simplest solution here is to replace the #ifdef with __maybe_unused
annotations, which lets the compiler do the right thing by itself.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 0184165b ("ravb: add sleep PM suspend/resume support")
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>