fix agc reset on SX126x, SX1276 & LR11x0 chips

[weebl2000]

• tried to simply explicitly call sleep briefly to reset AGC -> not sufficient it seems
• do more thorough reset using Calibrate (0x7F) -> success
• reset noise_floor so firmware shows accurate values
• so far it seems to work great on a Heltec v4
• should work on all SX126x based chips by using Calibrate(0x7F)
• shoudl work on all LR11x0 based chips by using Calibrate(0x3F)
• should work on SX1276 chips too, calling sleep there should be sufficient - SX1276's AGC is simpler — just LNA gain stepping. Sleep mode powers down the entire analog frontend, and re-entering RX restarts the AGC from G1 (max gain).
• default calibration is in 902-928 MHz range, made sure to follow up with actual configured frequency
• build firmware for yourself here: https://mcimages.weebl.me/?commitId=fixagcreset

fixes meshcore-dev#1716 - see discussion there for initial test results

I've referenced SX1262 datasheet to implement the calibration reset.

• p25 4.3 ,64-65 table 12-1 listing ΣΔ ADC architecture
• p55 table 9-1 sleep enable vs stdby_rc enable
• p55 9.2 states which blocks can be calibrated
• p61 9.2.1 calibrate specific freq
• p66 warm sleep restores register state confirms we need more aggressive Calibrate(0x7F) reset
• p73 total calibration time is 3.5 ms and must be launched in STDBY_RC mode

For LR1110 similar implementation, the 'AGC' is completely black boxed behind firmware so you can only reset it by resetting registers.

LR1110 datasheet page 9 section 1.2.1 states:
•Air interface fully compatible with the SX1261/2/8 family

Also see LR1110 user docs

So reset is very similar.

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Mirror of meshcore-dev#1743