iwlwifi: add missing type declaration

The iwl-debug.h header relies in implicit inclusion of linux/device.h and
we get a lot of warnings without that:

drivers/net/wireless/intel/iwlwifi/iwl-debug.h:44:23: error: 'struct device' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
 void __iwl_err(struct device *dev, bool rfkill_prefix, bool only_trace,
                       ^~~~~~
In file included from drivers/net/wireless/intel/iwlwifi/iwl-eeprom-read.h:66:0,
                 from drivers/net/wireless/intel/iwlwifi/iwl-eeprom-read.c:68:
drivers/net/wireless/intel/iwlwifi/iwl-trans.h: In function 'iwl_trans_tx':
drivers/net/wireless/intel/iwlwifi/iwl-trans.h:1030:348: error: passing argument 1 of '__iwl_err' from incompatible pointer type [-Werror=incompatible-pointer-types]
   IWL_ERR(trans, "%s bad state = %d\n", __func__, trans->state);
                                                                                                                                                                                                                                                                                                                                                            ^
In file included from drivers/net/wireless/intel/iwlwifi/iwl-eeprom-read.c:67:0:
drivers/net/wireless/intel/iwlwifi/iwl-debug.h:44:6: note: expected 'struct device *' but argument is of type 'struct device *'
 void __iwl_err(struct device *dev, bool rfkill_prefix, bool only_trace,
      ^~~~~~~~~

The easiest workaround is to just declare 'struct device' before its first use,
rather than including the entire header file.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 21cb3222 ("iwlwifi: decouple PCIe transport from mac80211")
Acked-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/intel/iwlwifi/iwl-debug.h

@@ -41,6 +41,7 @@ static inline bool iwl_have_debug_level(u32 level)
 #endif
 }
 
+struct device;
 void __iwl_err(struct device *dev, bool rfkill_prefix, bool only_trace,
 		const char *fmt, ...) __printf(4, 5);
 void __iwl_warn(struct device *dev, const char *fmt, ...) __printf(2, 3);