ath9k_common: clarify and correct jumbogram processing

Jumbograms are frames put together linked together through
more than one descriptor. For example ath9k_htc will use this
to send from the target a large frame split up into 2 or more
segments. The driver then would be in charge of putting the
frame back together.

When jumbograms are constructed the rx_stats->rs_more will
bet set and rx_stats->rs_status will not have any valid content
as the actual status will only be avialable at the end of
the chained descriptors.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>