mac80211: use listen interval 5 as default

Currently if a driver does not set hw.max_listen_interval a listen
interval of 1 is negotiated with the AP. Thus, the AP could drop
buffered frames for us after just one beacon interval which can
easily happen with the current powersave and scan implementation.
To avoid this issue increase the default interval to 5 which should
be a reasonable safe default.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>