usb: gadget: ether: Perform board initialization from ethernet gadget driver

Ethernet gadget driver can be used both by both SPL and u-boot. Since
usb_eth_init() is the entry point for ethernet gadget driver, perform
board initialization there. Also perform the cleanup in usb_eth_halt.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>