MAINTAINERS: update s390/net

Remove Thorsten Winkler as maintainer and add Aswin Karuvally as reviewer.

Thank you Thorsten for your support, welcome Aswin!

Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Acked-by: Thorsten Winkler <twinkler@linux.ibm.com>
Acked-by: Aswin Karuvally <aswin@linux.ibm.com>
Link: https://patch.msgid.link/20250813111633.241111-1-wintera@linux.ibm.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>