iwlwifi: move ucode loading to op_mode

uCode loading belongs to the op_mode, as it
is dependent on various things there and the
commands sent during it are specific to it.
Move the prototypes to iwl-agn.h to indicate
this. To make this possible, also move all
the calibration handling (which is op_mode
dependent after all).

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