Convert the mute LED handling in Conexant codec to the new vmaster
mute helper.  A point to be cautiously handled is that the value
passed to the callback is inverted; the vmaster passes "enabled"
(0 = mute), while LED classdev passes "brightness" (1 = mute).

Also the assignment of the default vmaster hook is moved at a later
point after the mute hook is set up.  This assures no nested hook.

Finally, since we enable the mute-LED kcontrols always in the helper
side, the extra vmaster_mute_enum flag set up is dropped.
Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20200618110842.27238-11-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>