MAINTAINERS: Add s390 networking drivers to NETWORKING DRIVERS

MAINTAINERS: Add s390 networking drivers to NETWORKING DRIVERS

These files are already correctly covered by the S390 NETWORKING DRIVERS
section. In practice commits for these drivers feed into the Networking
subsystem. So it seems appropriate to also list them under NETWORKING
DRIVERS.

This aids developers, and tooling such as get_maintainer.pl
alike to CC patches to all the appropriate people and mailing lists.
And is in keeping with an ongoing effort for NETWORKING entries
in MAINTAINERS to more accurately reflect the way code is maintained.

Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250417-ism-maint-v1-2-b001be8545ce@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>