ALSA: emu10k1: Fix emu1010 dock attach check

The emu1010_firmware_thread() checks the previous dock status, but a
wrong register is recorded as the last status when the dock is plugged
in.  Usually this isn't a big issue since this value gets overwritten
by the next loop after one second.  But when a dock is unplugged
immediately after plugging, it means essentially missing undock
handling.

This patch addresses it by remembering the correct register value.

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