iwlwifi: select MAC80211_LEDS conditionally

MAC80211_LEDS depends on LEDS_CLASS=y or LEDS_CLASS=MAC80211,
add condition to enable it in iwlwifi/Kconfig to avoid below
compile warning when LEDS_CLASS was set to m:

WARNING: unmet direct dependencies detected for MAC80211_LEDS
  Depends on [n]: NET [=y] && WIRELESS [=y] && MAC80211 [=y] && (LEDS_CLASS [=m]=y || LEDS_CLASS [=m]=MAC80211 [=y])
  Selected by [m]:
  - IWLWIFI_LEDS [=y] && NETDEVICES [=y] && WLAN [=y] && WLAN_VENDOR_INTEL [=y] && IWLWIFI [=m] && (LEDS_CLASS [=m]=y || LEDS_CLASS [=m]=IWLWIFI [=m]) && (IWLMVM [=m] || IWLDVM [=m])

Signed-off-by: Liwei Song <liwei.song@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>