net: ethernet: lantiq_etop: fix double free in detach

The number of the currently released descriptor is never incremented
which results in the same skb being released multiple times.

Fixes: 504d4721

 ("MIPS: Lantiq: Add ethernet driver")
Reported-by: Joe Perches <joe@perches.com>
Closes: https://lore.kernel.org/all/fc1bf93d92bb5b2f99c6c62745507cc22f3a7b2d.camel@perches.com/
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20240708205826.5176-1-olek2@wp.pl
Signed-off-by: Jakub Kicinski <kuba@kernel.org>