ALSA: snd-usb-caiaq: Restore 'Control vinyl' input mode on A4DJ

This feature was undocumented on early A4DJ units. It is indicated
by lighting both the 'line' and 'phono' lamps at the same time.
Newer units document this and the newer Windows drivers enable this
for all units, so restore the functionality.

This patch simplifies the code and changes the mode mapping to match
the A8DJ, favouring simpler code and consistency over keeping the
existing mapping.

Both 'Control vinyl' and 'Phono' input modes enable the hardware
preamp. The difference is the input impedance.

This reverts commit 9a9527ed.

Acked-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Mark Hills <mark@pogo.org.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>