iwlwifi: clean up last_phy_res

The last_phy_res[100] variable is used in an odd
way. The first byte of it is used as a flag, and
the rest as the data. Thus, the array need only
be 61 bytes, since it is just the flag and a
struct iwl_rx_phy_res (which is 60 bytes).

Clean this up by splitting the variable into two:
last_phy_res and last_phy_res_valid, using correct
types for both (struct and bool). While doing all
this also move the variables to the _agn part of
the hw-specific union since they only apply to
A-MPDUs.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>