iwlwifi: use iwl_hwrate_get_mac80211_idx where appropriate

For HT packets, mac80211 expects the rate_idx to be an MCS number, which is the
lower byte of rate_n_flags.  However, iwl_hwrate_to_plcp_idx takes the MCS
number and reduces it down to the range 0-8 (6 to 60 Mbps), removing the bits
that signify multiply streams, HT40 Duplicate mode, or unequal modulation.
This version is used for various internal purposes through the driver.

Add the function iwl_hwrate_get_mac80211_idx, an alternate version which takes
the rate and the band and returns the mac80211 index (MCS, for HT packets, and
PLCP rate, for legacy packets).

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>