MAINTAINERS: adjust file entry of the oa_tc6 header

Commit aa58bec0 ("net: ethernet: oa_tc6: implement register write
operation") adds two new file entries to OPEN ALLIANCE 10BASE-T1S MACPHY
SERIAL INTERFACE FRAMEWORK. One of the two entries mistakenly refers
to drivers/include/linux/oa_tc6.h, whereas the intent is clearly to refer
to include/linux/oa_tc6.h.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains about a
broken reference. Adjust the file entry to the intended location.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>