mac80211: fix beacon interval value

This patch fixes setting beacon interval

1. in register_hw it honors value requested by the driver
2. It uses default 100 instead of 1000 or 10000. Scanning for beacon
interval ~1sec and above is not sane

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>