MAINTAINERS: Add ism.h to S390 NETWORKING DRIVERS

MAINTAINERS: Add ism.h to S390 NETWORKING DRIVERS

ism.h appears to be part of s390 networking drivers
so add it to the corresponding section in MAINTAINERS.

This aids developers, and tooling such as get_maintainer.pl
alike to CC patches to 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-v1-1-b001be8545ce@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>