atm: remove the unused change_qos device operation

atm: remove the unused change_qos device operation

atmdev_ops::change_qos() was the hook for renegotiating the traffic
parameters of an already-connected VCC, driven from SO_ATMQOS on a
connected socket (and previously from the SVC as_modify path, now gone).
None of the ATM drivers left in tree implement it - solos-pci only listed
change_qos = NULL - so atm_change_qos() always returned -EOPNOTSUPP.

Drop the operation and return -EOPNOTSUPP directly.

Link: https://patch.msgid.link/20260615194416.752559-7-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>