iwlwifi: refactor rxon time command

This patch refactors rxon time command. It removes the usage of union tsf
in favor of u64 value and hopefully makes code more readable.  There are
no functional changes in this patch.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>