When setting tx power in sysfs, check against max channel tx power
limit instead of IWL_TX_POWER_TARGET_POWER_MAX.

Different devices have different max tx power limit; using
IWL_TX_POWER_TARGET_POWER_MAX can excess the limitaion and give wrong
information.
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>