usb: gadget: ether: Inline functions used once

These functions here are only ever called once since drop of non-DM
networking code. Inline them. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Tested-by: Tom Rini <trini@konsulko.com>
Tested-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Tom Rini <trini@konsulko.com>