iwlwifi: make MVM and DVM depend on MAC80211

It's not the iwlwifi module that depends on mac80211, but iwlmvm and
iwldvm.  To reflect this better, make MVM and DVM Kconfig options
depend on MAC80211 instead.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>