iwlwifi: Make injection of non-broadcast frames work again

Commit 1ccb84d87d04df3c76cd4352fe69786d8c7cf016 by Wey-Yi Guy
("iwlwifi: clean up unused NL80211_IFTYPE_MONITOR for Monitor mode")
broke injection of non-broadcast frames to unassociated stations
(causing a SYSASSERT for all such injected frames), due to injected
frames no longer automatically getting a broadcast station ID assigned.
This patch restores the old behavior, fixing the aforementioned
regression.

Also, consistently check for IEEE80211_TX_CTL_INJECTED instead of
iwl_is_monitor_mode in the TX path, as TX_CTL_INJECTED specifically
means that a given packet is coming from a monitor interface, while
iwl_is_monitor_mode only shows whether a monitor interface exists
on the device.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Acked-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>