hvc_console: make the ops pointer const.

This is nicer for modern R/O protection.  And noone needs it non-const, so
constify the callers as well.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: linuxppc-dev@ozlabs.org