ALSA: serial-generic: remove shared static buffer

If multiple instances of this driver are instantiated and try to send
concurrently then the single static buffer snd_serial_generic_tx_work()
will cause corruption in the data output.

Move the buffer into the per-instance driver data to avoid this.

Signed-off-by: John Keeping <jkeeping@inmusicbrands.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>