cfg80211: add missing dependency to CFG80211 suboptions

New options introduced by the patch this fixes are still
enabled even if CFG80211 is disabled.

.config:
    # CONFIG_CFG80211 is not set
    CONFIG_CFG80211_REQUIRE_SIGNED_REGDB=y
    CONFIG_CFG80211_USE_KERNEL_REGDB_KEYS=y
    # CONFIG_LIB80211 is not set

When CFG80211_REQUIRE_SIGNED_REGDB is enabled, it selects
SYSTEM_DATA_VERIFICATION which selects SYSTEM_TRUSTED_KEYRING
that need extract-cert tool. extract-cert needs some openssl
headers to be installed on the build machine.

Instead of adding missing "depends on CFG80211", it's
easier to use a 'if' block around all options related
to CFG80211, so do that.

Fixes: 90a53e44

 ("cfg80211: implement regdb signature checking")
Signed-off-by: Romain Naour <romain.naour@gmail.com>
[touch up commit message a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>