rawmidi: substream_status: use uint64_t for avail and xrun properties

rawmidi: substream_status: use uint64_t for avail and xrun properties

The avail and xrun members in struct snd_rawmidi_params are defined as
size_t type. The size_t is long type, thus the size is different between
32 bit ABI and 64 bit ABI. This commit uses uint64_t type for the
properties so that the size is the same as these two ABIs.

Fixes: 2371b927816d ("rawmidi: substream_status: add properties")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>