iwlwifi: rename iwl_remove to iwl_op_mode_dvm_stop

iwl_remove stops the wifi flows, so rename.
Moreover, we can possibly stop the wifi flows even if the driver
is statically compiled in the kernel, so remove the __devexit pragma.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>