s390-ccw: set cp_receive mask only when needed and consume pending service irqs

s390-ccw: set cp_receive mask only when needed and consume pending service irqs

It is possible while waiting for multiple types of external
interrupts that we might have pending irqs remaining between
irq consumption and irq-type disabling. Those interrupts
could potentially propagate to the guest after IPL completes
and cause unwanted behavior.

As it is today, the SCLP will only recognize write events that
are enabled by the control program's send and receive masks. To
limit the window for, and prevent further irqs from, ASCII
console events (specifically keystrokes), we should only enable
the control program's receive mask when we need it.

While we're at it, remove assignment of the (non control program)
send and receive masks, as those are actually set by the SCLP.

Signed-off-by: Collin L. Walling <walling@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>