ALSA: line6: Improve line6_read/write_data() interfaces

The address cannot be negative so make it unsigned.  Also, an unsigned
int is always sufficient for the length, so no need to overdo it with a
size_t.  Finally, add in range checks to see if the values passed in
actually fit where they are used.

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>