usb: remove not used variable in usb_ether_curr_dev

grepping for usb_ether_curr_dev in u-boot source code shows

$ grep -r usb_ether_curr_dev .
./cmd/usb.c:static int __maybe_unused usb_ether_curr_dev = -1; /* current ethernet device */
$

only declared but never used, so it can safely
removed from code.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>