mac80211: Fix config variable name for dtim_period

The vif option dtim_period was accidently renamed dtim_interval in r38988
("netifd: add wireless configuration support and port mac80211 to the new
framework"). This is wrong and makes the dtim_period/dtim_interval a dead
option because the rest of the config generation code still uses dtim_period.

Reported-by: Jeppe Ledet-Pedersen <jlp@steinwurf.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41557 3c298f89-4303-0410-b956-a3cf2f4a3e73