eth: pse: add missing static inlines

build bot reports missing 'static inline' qualifiers in the header.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 18ff0bcd

 ("ethtool: add interface to interact with Ethernet Power Equipment")
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20221004040327.2034878-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>