ath9k: disable periodic entropy gathering (keep initial) to fix some stability issues

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r41952

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42010 3c298f89-4303-0410-b956-a3cf2f4a3e73