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>

Author: SandyWinter

MAINTAINERS

@@ -22174,7 +22174,7 @@ F:	arch/s390/mm
 
 S390 NETWORK DRIVERS
 M:	Alexandra Winter <wintera@linux.ibm.com>
-M:	Thorsten Winkler <twinkler@linux.ibm.com>
+R:	Aswin Karuvally <aswin@linux.ibm.com>
 L:	linux-s390@vger.kernel.org
 L:	netdev@vger.kernel.org
 S:	Supported