coresight: stm: Fix missing disable flag when disabling STM

stm_disable function must set the drvdata->enable flag to false while
disabling the STM which is missing. Fix the enable flag to false
in stm_disable function.

Change-Id: Iec8df425bcbf44faa5d01051f5f9ae60c00e88f5
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>