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>

SVN-Revision: 41557