iwlagn: default to MAX_UCODE_BEACON_INTERVAL in iwl_adjust_beacon_interval

Default to MAX_UCODE_BEACON_INTERVAL if the output of
iwl_adjust_beacon_interval would otherwise be zero.  This prevents a
division by zero on my iwl5300-equipped Lenovo T400 with kernels that
include "mac80211: use cfg80211s BSS infrastructure".

This patch is a bit of a hack -- I'm not sure why iwl_setup_rxon_timing
is giving iwl_adjust_beacon_interval a zero input (which is the only way
it would output zero).  I would be happy to have a better fix.  But for
now, this makes my box boot...

Signed-off-by: John W. Linville <linville@tuxdriver.com>