ALSA: line6: pod: Rewrite complex timer & work combo with a delayed work

The POD driver had a complex staged startup procedure using both timer
and work.  This patch simplifies it via a single delayed work with the
reduced stages.

Now basically only two intermediate stages:
- POD_STARTUP_VERSIONREQ:
  requesting the version information and the process_message callback
  triggers the next stage,
- POD_STARTUP_SETUP:
  registering the actual card object.

Signed-off-by: Takashi Iwai <tiwai@suse.de>