[ALSA] usb-audio - SB Live24-External better handling

This patch improves support for 'SB Live 24-bit Extarnal' USB card.
1) This card can go into muted state when a headphones connected or
disconnected. So notify mixer about changes in headphone jack.
2) Add LED controls and procfs support just as in similar Audigy 2 NX card.
3) Rename 'PCM Capture' conrol to 'Mic Capture' to reflect reality:
the card may adjust microphone input level only.

Signed-off-by: Timofei Bondarenko <tim@ipi.ac.ru>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>