aplay: Introduce and use xwrite helper

aplay: Introduce and use xwrite helper

Write can return less then requested bytes, but we treat this as
an error thus ending up with confusing error messages.

Fix this by introducing xwrite helper, which makes sure all bytes
are written or an error is returned.

With this patch an usecase where disk is filled by recording will
print:
$ /mnt/msc/audio.wav: No space left on device

instead of random messages like:

$/mnt/msc/audio.wav: No such file or directory

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>