net: stmmac: fix stmmac_simple_pm_ops build errors

The kernel test robot reports that various drivers have an undefined
reference to stmmac_simple_pm_ops. This is caused by
EXPORT_SYMBOL_GPL_SIMPLE_DEV_PM_OPS() defining the struct as static
and omitting the export when CONFIG_PM=n, unlike DEFINE_SIMPLE_PM_OPS()
which still defines the struct non-static.

Switch to using DEFINE_SIMPLE_PM_OPS() + EXPORT_SYMBOL_GPL(), which
means we always define stmmac_simple_pm_ops, and it will always be
visible for dwmac-* to reference whether modular or built-in.

Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Signed-off-by: Russell King (Oracle) <[email protected]>
Reviewed-by: Andrew Lunn <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Russell King (Oracle)

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -8024,8 +8024,9 @@ int stmmac_resume(struct device *dev)
 }
 EXPORT_SYMBOL_GPL(stmmac_resume);
 
-EXPORT_GPL_SIMPLE_DEV_PM_OPS(stmmac_simple_pm_ops, stmmac_suspend,
-			     stmmac_resume);
+/* This is not the same as EXPORT_GPL_SIMPLE_DEV_PM_OPS() when CONFIG_PM=n */
+DEFINE_SIMPLE_DEV_PM_OPS(stmmac_simple_pm_ops, stmmac_suspend, stmmac_resume);
+EXPORT_SYMBOL_GPL(stmmac_simple_pm_ops);
 
 #ifndef MODULE
 static int __init stmmac_cmdline_opt(char *str)