s390/cpum_sf: Assign error value to err variable

When starting the CPU Measurement sampling facility using
qsi() function, this function may return an error value.
This error value is referenced in the else part of the
if statement to dump its value in a debug statement.
Right now this value is always zero because it has not been
assigned a value.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/perf_cpum_sf.c

@@ -1007,7 +1007,8 @@ static void cpumsf_pmu_disable(struct pmu *pmu)
 	}
 
 	/* Save state of TEAR and DEAR register contents */
-	if (!qsi(&si)) {
+	err = qsi(&si);
+	if (!err) {
 		/* TEAR/DEAR values are valid only if the sampling facility is
 		 * enabled.  Note that cpumsf_pmu_disable() might be called even
 		 * for a disabled sampling facility because cpumsf_pmu_enable()