nl82011: clarify interface combinations wrt. channels

Thinking about MLD (Draft 802.11be) now, it's clear that
we'll have new limitations where different stations (or
links) must be on _different_ channels (or in different
frequency ranges even.) Clarify that the current limit of
multiple channels is a maximum and the same one is OK.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20211201110924.2816d91b6862.I2d997abd525574529f88e941d90aeb640dbb1abf@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>