ASOC TFA9890: cancel monitor thread in sync stereo mute function

status monitor thread can still run after, amplifiers are
disabled and muted in stereo sync mute fuction, causing unintended
amp re initialization leading to volume inconsistencies.

Change-Id: Ie1036427242ebfca070465da2ca4a9ec6bb7ce06
Signed-off-by: Prabhu Annabathula <prabhu.annabathula@motorola.com>