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>