iwlwifi: fix sparse warning about wrong enum for band parameter

drivers/net/wireless/iwlwifi/iwl-scan.c:386:27: warning: mixing different enum types
drivers/net/wireless/iwlwifi/iwl-scan.c:386:27:     int enum nl80211_band  versus
drivers/net/wireless/iwlwifi/iwl-scan.c:386:27:     int enum ieee80211_band
drivers/net/wireless/iwlwifi/iwl-scan.c:435:57: warning: mixing different enum types
drivers/net/wireless/iwlwifi/iwl-scan.c:435:57:     int enum ieee80211_band  versus
drivers/net/wireless/iwlwifi/iwl-scan.c:435:57:     int enum nl80211_band
drivers/net/wireless/iwlwifi/iwl-scan.c:474:53: warning: mixing different enum types
drivers/net/wireless/iwlwifi/iwl-scan.c:474:53:     int enum ieee80211_band  versus
drivers/net/wireless/iwlwifi/iwl-scan.c:474:53:     int enum nl80211_band
drivers/net/wireless/iwlwifi/iwl-scan.c:588:72: warning: mixing different enum types
drivers/net/wireless/iwlwifi/iwl-scan.c:588:72:     int enum ieee80211_band  versus
drivers/net/wireless/iwlwifi/iwl-scan.c:588:72:     int enum nl80211_band

Signed-off-by: John W. Linville <linville@tuxdriver.com>