iwlwifi: remove get_cmd_string

The command strings are needed through the layers for
debug and error messages, but can differ with opmode.
As a result, we need to give the command names to the
transport layer as configuration.

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