iwlagn: refactor up path

Starting the device consists of many things,
refactor out enabling the hardware and also
return -ERFKILL when the rfkill signal is
found to be asserted (which makes more sense
anyway, but is also required now to make the
__iwl_up function return right away.)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>