ALSA: oxfw: some signedness bugs

This code tends to use unsigned variables by default and it causes
signedness bugs when we use negative variables for error handling.
The "i" and "j" variables are used to iterated over small positive
values and so they should be type "int".  The "len" variable doesn't
*need* to be signed but it should be signed to make the code easier to
read and audit.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>