iwlwifi: mvm: don't send identical PHY_CTXT_CMD

When we have an AP which supports HT and a single HT
station is connected, we change the min_width from
NL80211_CHAN_WIDTH_20_NOHT to NL80211_CHAN_WIDTH_20. This
of course has no implication on the channel width but still
sends a command to the firmware.
Remember the last width that was sent and refrain from
sending unnecessary commands to the firmware.

Sending a PHY_CTXT_CMD to the firmware has a cost since it
recalculates the presence on the medium and because of that
it closes the transmit queues for a short while.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>