drivers/hv: remove obsolete TODO and fix misleading typo in comment

Removes an obsolete TODO in the VMBus module and fixes a misleading typo
in the comment for the macro MAX_NUM_CHANNELS, where two digits have been
twisted.

Signed-off-by: Stefan Eschenbacher <stefan.eschenbacher@fau.de>
Co-developed-by: Max Stolze <max.stolze@fau.de>
Signed-off-by: Max Stolze <max.stolze@fau.de>
Link: https://lore.kernel.org/r/20201206104850.24843-1-stefan.eschenbacher@fau.de
Signed-off-by: Wei Liu <wei.liu@kernel.org>