MAINTAINERS: adjust file entry for of_net.c after movement

Commit e330fb14

 ("of: net: move of_net under net/") moves of_net.c
to ./net/core/, but misses to adjust the reference to this file in
MAINTAINERS.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains:

   warning: no file matches    F:    drivers/of/of_net.c

Adjust the file entry after this file movement.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20211016055815.14397-1-lukas.bulwahn@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>