[ALSA] rawmidi: adjust runtime->avail when changing output buffer size

Modules: RawMidi Midlevel

When the output buffer size is changed, runtime->avail must be
adjusted to the new size; otherwise, draining of a bigger buffer would
appear to fail because avail does not reach buffer_size.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>