cfg80211: add more comments for ap_isolate in bss_parameters

cfg80211: add more comments for ap_isolate in bss_parameters

The value of struct bss_parameters::ap_isolate will be -1, 0 or 1.
The value -1 means not to change. To prevent developers from thinking
ap_isolate is only 0 or 1, I add more comments on it.

Signed-off-by: Wright Feng <wright.feng@cypress.com>
Reviewed-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200908060157.98846-1-wright.feng@cypress.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>