iwlwifi: virtualize the op_mode

Define the op_mode as an interface with its ops. All the functions
of the op_mode are "private", but its ops is made public in
iwl-op-mode.h.
The drv object starts the op_mode by using the start function in the
public ops.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>