Telephony: Modify code to check PUK code length is always 8.

As per 3GPP TS 31.101 Release 11 section 9.6, the number of digits
for UNBLOCK PIN (PUK) is always 8.

CRs-Fixed: 596207

Change-Id: I55dc9f0a701410d8405026e625a5f2a98a9f712c