PHY: bugfix: compute start/end of burst correctly for TDD

Start of burst is to start TDD DL transmission in the driver
(tested with USRP for the moment).

End of burst is to stop DL transmission.

Start of burst can only happen for a DL subframe when the
previous subframe was an UL subframe.

End of burst can only happen for an S subframe.

It's impossible for a subframe to be both the start of a burst
and the end of a burst.

This can be checked with eg. http://niviuk.free.fr/lte_resource_grid.html
(or reading the specs).