Expected PHR value fixed

  - as per TS 38.321 section 6.1.3.8, power headroom is an unsigned
    integer of 6 bits which translates to
[POWER_HEADROOM_0,...,POWER_HEADROOM_63]
  - as per TS 38.133 section 10.1.17.1.1, each POWER_HEADROOM_x
    represents the range of values in dB
  - "phr" parameter in MAC stats represent the UE power headroom in dB
    => therefore, it is a signed integer

Future KPM measurement extension: as per TS 28.552 section 5.1.1.26.1,
the BinX is within the range [POWER_HEADROOM_0,...,POWER_HEADROOM_63],
so the max number of bins is 64 but please note that it can be < 64 which is left to implementation;
each bin has a value that represents the number of UEs within the dB range of each bin
=> please note that this is a node level measurement