doc: Fix comparison operator

Align the documentation with the include/linux/etherdevice.h ,
which is where this example comes from. The return value from
the check was inverted in the documentation.

Signed-off-by: Olaf Mandel <o.mandel@menlosystems.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Tom Rini <trini@ti.com>