wl12xx: fix "JOIN while associated" commentary

Issuing multiple JOIN commands to the wl12xx's firmware, while
we're associated, might have undesired implications, so the driver
prints a message when that happens, and warn developers who check
out the source.

Update the commentary in order to consider the one valid scenario
where this can happen: roaming.

Cautiously keep the message for now, until we either gain confidence
there are no unintentional JOIN-while-associated events, or until
we move to the new multi-role fw who solves this multiple-join issue
for good.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>