iwlwifi: mvm: fix tof.h header guard

Commit ce792918 ("iwlwifi: mvm: add basic Time of Flight (802.11mc
FTM) support") created drivers/net/wireless/iwlwifi/mvm/tof.h with a
broken header guard:

    #ifndef __tof
    #define __tof_h__

    ...

    #endif /* __tof_h__ */

Use __tof_h__ in the first line.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>