s390x: css: simplifications of the tests

In order to ease the writing of tests based on:
- interrupt
- enabling a subchannel
- using multiple I/O on a channel without disabling it

We do the following simplifications:
- the I/O interrupt handler is registered on CSS initialization
- We do not enable again a subchannel in senseid if it is already
  enabled
- we add a css_enabled() function to test if a subchannel is enabled

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/kvm/1615545714-13747-3-git-send-email-pmorel@linux.ibm.com/


Signed-off-by: Janosch Frank <frankja@linux.ibm.com>