Reading and writing back the tsf value via tsf is too slow if one wants
to make small increments to this timer.  With this change you can use
the syntax "+=<some value>" or "-=<some value>" to add or substract a
value from the tsf counter.
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>