iwlwifi: mvm: refactor beacon template command code

We currently support 3 different versions of the beacon template
command and the code does some tricks in order to reuse what is
possible across these versions.  But it is a bit complicated to read
and soon there will be one more variation that the driver needs
implement, which would complicate it even further.

Refactor the way we send beacon template commands, which increases the
code size a bit, but makes it much easier to read.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>