iwlwifi: dynamically determine lib_ops

Having the pointer to lib_ops in the config
makes it impossible to split the driver into
different modules. Determine the ops based on
the device family enumeration to get rid of
the direct pointer.

Also move all the opmode specific code from
the iwl-[1256]000.c files into a new file
iwl-agn-devices.c so that the former only
have configuration data now.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>