plca.c: fix obvious mistake in checking retval

Revert a wrong fix that was done during the review process. The
intention was to substitute "if(ret < 0)" with "if(ret)".
Unfortunately, the intended fix did not meet the code.
Besides, after additional review, it was decided that "if(ret < 0)"
was actually the right thing to do.

Fixes: 8580e16c

 ("net/ethtool: add netlink interface for the PLCA RS")
Signed-off-by: Piergiorgio Beruto <piergiorgio.beruto@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/f2277af8951a51cfee2fb905af8d7a812b7beaf4.1673616357.git.piergiorgio.beruto@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>