refactor(radio): unified battery and RTC voltage

[raphaelcoeffic]

Summary of changes:

• use the same voltage conversion formula for all targets
• fix RTC battery bridge on H7 targets

[philmoz]

😕

• TX16S - 275.99V (should be 7.68V)
• T15 - 358.14V (should be 8.02V)
• EL18 - 527.08V (should be 4.13V)

[raphaelcoeffic]

Seems I forgot to adapt for how BATTERY_DIVIDER worked ($R1$ and $R2$ are unknown here):

$$BATTERY\_DIVIDER = ADC_{max} * \frac{R1}{R1 + R2} * \frac{10}{V_{ref}}$$

whereby $ADC_{max} = 2048$, as we're using filtered ADC values.

However, what we use now is something like this:

$$VBAT\_DIVIDER = \frac{R1 + R2}{R1}$$

So that gives us:

$$BATTERY\_DIVIDER = ADC_{max} * \frac{1}{VBAT\_DIVIDER} * \frac{10}{V_{ref}}$$

and finally:

$$VBAT\_DIVIDER = \frac{ADC_{max} * 10}{BATTERY\_DIVIDER * V_{ref}}$$

[philmoz]

T15 is good now, haven't checked the others yet.

[pfeerick]

After minor rebase snaffu...
EL18 - unchanged
X9D+ - only minor vbat change (approx 0.3v), RTC unchanged
MT12 - only minor vbat change (approx 0.3v), RTC did go up, but is basically flat, so that could be some voltage bleed.
T14 - only minor vbat change (approx 0.2v), RTC unchanged
T15 - unchanged