ao/wasapi: don't assume 32-bits == float

This was based on old restrictions WAVEFORMATEX restrictions
http://msdn.microsoft.com/en-us/library/windows/hardware/ff538799%28v=vs.85%29.aspx

With the new WAVEFORMATEXTENSIBLE, this is no longer a restriction.
ao/wasapi: don't try unsigned formats

before this led to static with --audio-format=u24 or presumably if the
media itself was unsigned.
ao/wasapi: show actual waveformat tried

also remove bogus ao_format
ao/wasapi: fix set_ao_format

before it was always using the ao->format to change bits. this was
broken if the initial ao->format was a float and for example
we wanted to try s24 instead.
ao/wasapi: get rid of WAVEFMT union

it only confused the issue.