MAINTAINERS/vfio-ccw: add Farhan and Eric, make Halil Reviewer

Eric and Farhan will help with maintaining vfio-ccw.

Cc: Cornelia Huck <cohuck@redhat.com>
Cc: Halil Pasic <pasic@linux.ibm.com>
Cc: Farhan Ali <alifm@linux.ibm.com>
Cc: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20181212145929.136522-1-borntraeger@de.ibm.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Acked-by: Farhan Ali <alifm@linux.ibm.com>
Acked-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>