pc-bios/s390-ccw: enable subchannel for IPL I/O devices

IPL should cause the IPL I/O device to become enabled. So when handling
the IPL program, we should set the E (Enable) bit. However, virtio-ccw
does not know whether it's dealing with an IPL device or not. Since
trying to perform I/O on a disabled device doesn't make any sense,
let's just always enable it. At the same time we can remove the
SCSW_FCTL_START_FUNC flag as it is ignored for msch anyway and did
not enable the device as intended.

Reported-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Signed-off-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
[remove superfluous flag]
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>