ath5k: reduce exported channel list

Claiming every available 5 ghz channel has a couple of negative
side-effects: scanning takes a long time, and the channel list
overflows the available buffer space for netlink commands,
resulting in:

    $ iw phy phy0 info
    command failed: No buffer space available (-105)

This patch adds a modparam so people who want to see all the channels
can do so by passing all_channels=1.  By default users will see a
smaller list of channels.  This also halves scan time, from 10 seconds
down to less than 5 when using world regulatory.

Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Reported-by: Simon Farnsworth <simon@farnz.org.uk>
Tested-By: Simon Farnsworth <simon@farnz.org.uk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>