coresight: Don't enable the source more than once from sysfs

If enable source more than once from sysfs, the refcount will be
larger than one. Then the source will not be able to be disabled
from sysfs next time.

Change-Id: I5a30618576ffa05f17354eff8b564cbf0b0bbee2
Signed-off-by: Mao Jinlong <jinlmao@codeaurora.org>