iwlwifi: fix TX cmd dma unmapping

This patch:
1. fixes command DMA unmapping, this might be visible only
	on platforms where DMA unmapping is no noop such as PPC64 (not tested)
2. attaches correctly high memory part of the host command buffer
3. changes structure of TFD TB
	instead of describing transmit buffer (TB) tuple it describes single
	TB and makes code more readable on price of one unaligned access
4. eliminates using of IWL_GET/SET_BITs for TFD handling
5. renames TFD structures to mach the HW spec
6. reduces iwl_tx_info size by reserving first TB to the host command

This patch should not have any visible effect on x86 32

This patch is rework of
iwlwifi: fix DMA code and bugs from
Johannes Berg <johannes@sipsolutions.net>

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>