iwlwifi: move firmware request into drv

Firmware request is a base driver flow,
it isn't related to any specific mode.
Move the code related to it into the
base driver file iwl-drv.c.

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>