Fixed the spam debug message

2147483647 is a valid RSSI value returning from the underlying
modem. Should not generate any error messages.

Fix: 399481218
Test: atest CtsTelephonyTestCases:SignalStrengthTest#testSignalStrength
Flag: EXEMPT debug message fix
Change-Id: I3038d2f842bfa7c7680d3843a3befb1e74d70f5e