issue 137 - fix RI bits decoding

RI bits are present only in some transmission modes.

For aperiodic reporting (the mode we do as of today),
36.213 7.2.1 (release 10) says:

    "RI is only reported for transmission modes 3 and 4,
    as well as transmission modes 8 and 9 with PMI/RI reporting"

This commit activates decoding of RI bits only for transmission
modes 3 and 4. 8 and 9 are not done today (as far as I know).