Fix signal strength for HFP in maemo6 telephony

Fix signal strength for HFP in maemo6 telephony

Signal strength in terms of bars units is updated by emited
SignalBarsChanged instead of SignalStrengthChanged from libscnet library.
It contains required number of bars to display in scale 0..5. Conversion
from percent or decibel units to bars is not needed and therefore removed.

Same holds when initializing by request of SignalBars, where bars are in
0..5 range. Conversion from percents is removed. Sanity check is done to
prevent -1 that may be returned when the property is unavailabe (modem
state off).

RSSI percents and dBs are neither present among SignalBarsChanged
properties nor requested during initialization and, therefore, removed.