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 <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508132051.a7hJXkrd-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202508132158.dEwQdick-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202508140029.V6tDuUxc-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202508161406.RwQuZBkA-lkp@intel.com/


Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uojpo-00BMoL-4W@rmk-PC.armlinux.org.uk


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