KVM: s390: fix instruction interception trace point

trace-cmd fails to parse the instruction interception trace point:
  "Error: expected type 5 but read 4
   failed to read event print fmt for kvm_s390_intercept_instruction"
The result is an unformatted string in the output, with a warning:
  "kvm_s390_intercept_instruction: [FAILED TO PARSE]..."

So let's add parentheses around the instruction parser macro to fix the format
parsing.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>