verify_tpm_measurements: Fix TPM compare() error

The script fails with the following:
Traceback (most recent call last):
  File "/bin/verify_tpm_measurements.py", line 92, in <module>
    compare()
  File "/bin/verify_tpm_measurements.py", line 82, in compare
    if flag == 1 :
UnboundLocalError: local variable 'flag' referenced before assignment

Assign a default value to flag and fix the logic to track a failure
on any line and report it.

Signed-off-by: Nhi Pham <nhi@os.amperecomputing.com>