ALSA: emu10k1 - Add capture boost mixer switch for Audigy

Due to the conversion (drop) from 24bit in the DSP to 16bit in AC97,
the maximum capture level on Audigy seems lower than it could be.

This patch adds a workaround to enable the artificial capture boost
switch.  When this switch is on, the whole analog capature level is
boost up.  However, this results in the lower capture resolution.

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