iwlagn: new RXON processing for modern devices

In order to simplify the flow, and make new
enhancements easier, separate out the RXON
processing for modern AGN (5000 and newer)
from RXON processing for the older 3945 and
4965 devices. Avoid changing these old ones
to avoid regressions and move their code to
a new file (iwl-legacy.c). 4965 gets the
commit_rxon that used to be common for all
AGN devices, but with removed PAN support.

The new RXON processing is more central and
does more work in committing, so that it is
easier to follow.

To make it more evident what is split out
for legacy, split the necessary operations
for that into a new struct iwl_legacy_ops.
Those parts that still exist in the new AGN
code don't need to be parametrized.

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>