iwlwifi: refactor packet reception code

This patch fixes a number of issues in iwl_rx_reply_rx and
iwl_pass_packet_to_mac80211.  These issues stem from the complexities of
managing two different types of packet commands for different hardware.

- Unify code handling rx_phy_res in SKB or cached to eliminate redundancy and
remove potential NULL pointer accesses
- Replace magic number with proper constant
- Optimize functions by moving early exit conditions before computation
- Comment code and improve some variable names
- Remove redundant computation in iwl_pass_packet_to_mac80211 by passing in the
correct, already-computed arguments.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>