intel_th: msu: Serialize enabling/disabling

In order to guarantee that readers don't race with trace enabling,
both should happen under the same mutex. Having two mutexes seems
like an overkill, considering that because of the above, they'll
have to be acquired together, around trace enabling and char device
opening.

This patch makes both buffer accesses and readers serialize on
msc::buf_mutex and makes sure that 'enabled' flag accesses are also
serialized on it.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Laurent Fert <laurent.fert@intel.com>