Fix calculation of RSSI in pucch_rx.c

Fix two issues with RSSI calculation in PUCCH:

• RSSI was calculated using partial sum instead of average of squared samples.
• There was a missing parentheses pair which causes the RSSI value to underflow.

The proper way to calculate power in dBFS is

let signal_energy = average of squared samples of received signal over resource elements, antennas and symbols under measurement

then:

rssi[dBFS] = -10log10(max_sample^2 / signal_energy),

or (to avoid division):

rssi[dBFS] = -(10log10(max_sample^2) - 10log10(signal_energy))